Theory Biosci. DOI 10.1007/s12064-016-0222-1

ORIGINAL PAPER

Sexual selection and the opportunity cost of free mate choice Menelaos Apostolou1

Received: 17 December 2015 / Accepted: 17 February 2016 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract The model of sexual selection under parental choice has been proposed to account for the control that parents exercise over their children’s mating decisions. The present paper attempts to formalize and advance this model with the purpose of providing a better understanding of how parental choice mandates the course of sexual selection. In particular, in the proposed formulation, free mate choice involves an opportunity cost which motivates parents to place their children’s mate choices under their control. When they succeed in doing so, they become a significant sexual selection force, as traits that appeal to parents in an in-law are selected and increase in frequency in the population. The degree of parental control over mating, and thus the strength of sexual selection under parental choice, is positively predicted by the size of the opportunity cost of free mate choice. The primary factors that affect the level of opportunity cost vary between society types, affecting the strength of parental choice as a sexual selection force. Keywords Sexual selection  Parental choice  Sexual selection under parental choice  Female choice  Mate choice

& Menelaos Apostolou [email protected] 1

University of Nicosia, 46 Makedonitissas Ave., 1700 Nicosia, Cyprus

Introduction Members of sexually reproducing species require access to the reproductive capacity of the opposite sex in order to be able to pass their genetic material to future generations. Consequently, traits which enable an individual to secure opposite sex mates, increase in frequency in the population by giving to those endowed with them reproductive benefits. This is the process of sexual selection, first described by Darwin (1871). By investing more in their offspring, females become the scarce reproductive resource to which males strive to gain sexual access (Trivers 1972). Thus, females are in a position where they can exercise mate choice, meaning that traits which turn males more likely to be chosen as mates give a reproductive advantage to those endowed with them and, therefore, increase in frequency in the population; in effect, sexual selection under female choice takes place (Andersson 1994). This model of sexual selection is the prevailing one in accounting for sexual selection in human species (Apostolou 2014c). The problem with this model is, however, that it is not consistent with the anthropological and historical records of pre-industrial societies, which indicate that women were not free to exercise mate choice, with spouses being selected for them by their parents (Apostolou 2014a). Recently, a different model of sexual selection has been proposed, in which parents constitute an important sexual selection force, and which provides a better account of the observed patterns of human mating across pre-industrial societies (Apostolou 2007, 2010, 2012, 2014c). The purpose of the present paper is to formalize and advance this model, and apply it in understanding how the strength of parental choice as a sexual selection force varies across different society types.

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The model of parental choice Parent-offspring conflict over mating Parents and children are genetically related but not genetically identical, which means that their genetic interests overlap but also diverge (Trivers 1974). This difference results into conflict between the two, with mate choice being one area where this conflict is manifested (Apostolou 2014b; Trivers 1974). A good example of this conflict constitutes the genetic quality. Prospective mates differ in their genetic quality: some carry more genetic mutations, while others have alleles which enable them to withstand better the challenges of the environment. Individuals are 0.50 related to their children and 0.25 related to their grandchildren, which indicates that it is beneficial to get mates and in-laws of good genetic quality, because doing so will lead to having children and grandchildren who have good chances of survival and reproduction. This reasoning indicates further that it is more beneficial for individuals to get mates of superior genetic quality than in-laws of superior genetic quality. The reason is that individuals have more to lose if, due to poor genetic quality, their children rather than their grandchildren suffer survival or reproductive penalties, since they are more closely related to the former than to the latter. In consequence, parents and children would agree that good genetic quality is important in a mate, but they would disagree on how important this trait is, with the latter ascribing more importance to it than the former (Apostolou 2008). Differential benefits are not a sufficient condition for conflict to exist between two parties. For conflict to exist, the choices one party makes need to inflict a cost to the other—if they do not, conflict will not arise. Both parents and children have an interest in getting a spouse and an inlaw, respectively, of good genetic quality; thus, if for instance children attract a spouse of superior genetic quality, their parents would not object to this choice since they also benefit from this trait. What they will object to, are the compromises that their children had made in other traits in order to get such a spouse, compromises which are not compensated by the superior genetic quality as this trait is less beneficial to them. In more detail, children are constrained by their own mate value with respect to the mate value of the individual they can attract (Li et al. 2002). That is, individuals looking for long-term mates cannot attract mates of much greater value than their own, because these mates would not be willing to enter in such a relationship as it would not be optimal for them to accept mates of a lower mate value to their own (Apostolou 2011a; Buss 2003). Therefore, mate choice inevitably involves compromises: to be able to keep

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a long-term partner, mate seekers have to accept mates who score lower in desirable traits than they would prefer. The asymmetrical fitness benefits that certain traits provide result into mate-seekers making compromises which are not to the best interest of their parents (Apostolou 2011a). For example, mate-seekers would have to compromise on traits such as social status and family background in order to get a mate of high genetic quality. Nevertheless, because this trait is not as beneficial in an inlaw, the accrued benefits will not balance the losses from the compromises in other desirable qualities. Thus, from the parents’ point of view, these compromises are not optimal, and if they could exercise mate choice for their children, they would prefer a different mix of trait, a mix that would involve less of good genetic quality and more of other traits. Consistent with this prediction, when children and their parents were given a budget of mate points for the purpose of designing a spouse and an in-law, respectively, children saved on traits, such as good family background and similar religious background in order to get more of good looks (a proxy of genetic quality) (Apostolou 2011a). Their parents, on the other hand, saved on beauty in order to get more of good family background and similar religious background. Children also saved more than their parents on traits that relate to the provision of resources, such as favorable social status, ambition, and industriousness. In consequence, different evolutionary pressures are exercised on in-law and mate preferences to diverge in order to enable each party to make optimal compromises (Apostolou 2008, 2011a). Empirical research has, so far, identified that good looks (a proxy of genetic quality) and an exciting personality are preferred more in a spouse than in an in-law, while good family background and similar religious background are preferred more in an in-law than in a spouse (Apostolou 2008, 2014c; Buunk et al. 2008; Perilloux et al. 2011). Overall, free mate choice involves an opportunity cost coming from diverging genetic interests (odiverging)—see Appendix for a full list of the factors in the model—which is what parents forgo when they allow their children to exercise mate choice freely on their own. To see how this cost materializes, consider the following example: take a man living in a pre-industrial society with an adult daughter of good genetic quality, who has reached marital age. In such a context, a prospective husband and his parents need to pay an amount of wealth to the father for the marriage to proceed, which is known as bridewealth (Goody and Tambiah 1973). Thus, the father can arrange a marriage for his daughter with a man of similarly good genetic quality in return for a certain bridewealth. Alternatively, he can arrange a marriage with a man of inferior genetic quality for a considerably higher bridewealth that

Theory Biosci.

the prospective spouse and his parents would be willing to pay to compensate for the low genetic quality. The second choice can potentially increase his fitness more than the first choice. More specifically, the father can use the bridewealth from the marriage of his daughter to pay the bridewealth necessary for getting additional wives for himself (Goody and Tambiah 1973). The children coming from these wives will increase his fitness, but such fitness gains need to be discounted by the fitness losses he will suffer from the poor genetic quality of his daughter’s children. Nevertheless, because the father is more closely related to his children than to his grandchildren, the fitness gains will exceed the fitness costs, resulting in a net increase in his fitness. This is not true, however, for his daughter, who is more closely related to her children than to her father’s children. If she was to exercise choice, she would prefer a man with superior genetic quality paying a lower bridewealth, as this choice would lead to her having children of superior genetic quality. It needs to be said at this point that the conflict does not have to be solely over genetic quality. For instance, if the father was not considering men of varying genetic quality for his daughter, but men of varying non-heritable parenting ability, his decision would be the same: he would compromise on parenting ability to get a higher bridewealth. Overall, if some potential men allow for alliances that would increase the father’s own access to mates, and hence may increase his reproductive success, he would put relatively more value on such men than his daughter would, when compared to men that only confer genetic or non-genetic benefits to his daughter. In consequence, if the father allows his daughter to exercise choice freely, he will suffer an opportunity costs which equals to the fitness gains he could have received by getting additional wives and having children with them, and he does not because of the mate choices of his daughter. This cost will be examined in more detail below. Diverging opportunity cost The opportunity cost of free mate choice results into evolutionary pressure exercised on parents to place the mate choices of their children under their control. In particular, parents who have a predisposition to control mating have a selective advantage over those who do not, as the former are likely to have in-laws who maximize their inclusive fitness, while the latter are likely to have in-laws who maximize their children’s inclusive fitness. When parents succeed in controlling mate choice, they become an important sexual selection force: Traits that make an individual more likely to be chosen as an in-law, are

favored and increase in frequency in the population (Apostolou 2007, 2010). The strength of sexual selection under parental choice is predicted by the strength of parental control over mating (m) which is predicted by the opportunity cost of free mate choice: m = odiverging. The higher the opportunity cost, the more beneficial will be for parents to control mate choice, the more effort they will divert in doing so, the more control they will exercise over mate choice, the stronger the sexual selection under parental choice will be. The opportunity cost depends positively on the benefit parents receive, in terms of desirable traits when they control mate choice, which is over and above the benefit they would get if their children were to exercise choice on their own (b). Also, since control over mating involves parents allocating resources such as time to chaperon their children and effort to punish them in order to align them with their will (see below), the (b) needs to be seen as a net benefit: it is the benefit discounted by the cost involved in parental effort to control mating. This benefit needs to be adjusted by its potential (f) to be converted into fitness benefits (Apostolou 2014b): f = potential for increasing fitness directly (i.e., by having children) ? potential for increasing fitness indirectly (i.e., by benefiting genetic relatives). Nevertheless, in order to receive this benefit, parents have to make compromises which are costly to their children (c). But this cost is also their cost because they are genetically related to their children. Accordingly, the (c) needs to be adjusted by the degree of genetic relatedness (r). In order to get a more precise measure of genetic relatedness, let’s call it adjusted r or ( r ), the (r) needs to be multiplied by the probability (p) that one is actually an individual’s genetic relative (Apostolou 2014b). Therefore, the net benefit of a mating deal to parents, and thus, the opportunity cost of not controlling mate choice becomes: odiverging ¼ b  f  c  r. The benefit that parents receive from controlling their children’s mate choices comes predominantly from gains in family background units, which mainly translates into benefits from political support and resources. Parents gain further in terms of resources as they will, potentially, compromise less on the resource acquisition capacity of their prospective in-laws, than their children for their prospective spouses (see Apostolou 2011a). Therefore, the level of the benefit (b) is a positive function of the level of available resources (a). For instance, parents make compromises in traits such as beauty, in order to forge a beneficial alliance with another family. If the other family controls more resources, the benefit of this alliance will be higher than if it controls fewer resources. The cost (c) is not a function of available resources (a), because when parents exercise choice, they inflict a cost to

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their children, predominantly in terms of beauty and exciting personality (Apostolou 2011a, 2014c). In particular, evidence indicates that beauty and exciting personality are valued less by parents than by their children (Apostolou 2014c; Buunk et al. 2008; Perilloux et al. 2011). Thus, when parents exercise choice, they would tend to compromise more on these traits than their children would wish, in order to get more of other desirable traits such as good family background (Apostolou 2011a). Lastly, the benefit parents can derive from controlling the mate choices of their children depends on the latter’s mate value (v). If children have high mate value (e.g., they are physically attractive), parents can extract more benefits than in the case when the children have low mate value. Yet, children of high mate value risk losing more from parents controlling their mate choices, whereas they could get more desirable traits, if they were to exercise choice on their own. Therefore this mate value (v) determines the level of the diverging opportunity cost: odiverging ¼ ðb  f  c  rÞ  v: The cost that parents can inflict on their children to align them with their will The level of the diverging opportunity cost of free mate choice depends on parents’ capacity to inflict a cost to their children in order to align them with their will (d). If parents can inflict no cost to their children, the latter would have no reason to allow the former to choose a spouse who is not optimal for them. In different words, when the (d) becomes zero, the converging opportunity cost of free mate choice becomes also zero, as parents cannot impose mates on their children. Therefore, this effect of (d) can be expressed as: o ¼ odiverging  d: The (d) is expected to be almost always positive because parents have several advantages in their hands, which are predominantly derived from the fact that they are older than their children. To begin with, during their lifetime parents have accumulated material resources, such as money and land, which are diverted to their children in the form of parental investment (w). These resources include also non-material ones, such as their capacity to provide physical protection. The more resources parents have control over, the more their children have to lose from defying them. Children’s younger age makes them less experienced in subsistence activities and less likely to have control over the means of producing wealth, such as land and cattle. Children are also less experienced than their parents in defending and protecting themselves from external threats. Accordingly, they are dependent on their parents’ resources, and the degree of this dependence (e) predicts

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positively the cost that parents can inflict on their children: the more dependent children are on their parents, the more cost parents can inflict on them by discontinuing parental investment. Note that the (w) and the (e) are treated as separate factors because, even if children are totally independent from their parents’ resources, they still have a lot to gain from these resources, which means that they are at risk to lose if their parents decide not to divert these resources to them. Furthermore, by being older, parents are, up to a point, physically stronger than their children. Accordingly, they can use this difference in strength, and they may also ask the assistance of physically strong relatives for this purpose, to impose their will by means of physical force (h). For instance, they can physically punish a daughter for undesirable sexual behavior, or for refusing an arranged marriage. Therefore, the cost that parents can inflict on their children is positively related to their ability to use physical force to impose their will. Apart from physical cost, parents can also inflict psychological cost to their children (j). For instance, parents can threaten their children with consequences, they can cry, look sad etc. (Apostolou 2013a; Sussman 1953). This capacity of parents does not stem from their older age, but the age difference between their children and them, can enhance it. For example, experience may enable parents to refine and enrich the manipulation tactics they can use on their children. Note that manipulation is viewed as a cost because it can have adverse fitness consequences for children. In particular, manipulation takes place when one party attempts to influence another party with the purpose of promoting the former party’s interests (Buss 1992). As the interests of parents and children are not completely aligned, effective manipulation of the latter from the former would translate into fitness losses for children and fitness gains for parents. For instance, children are manipulated into accepting mates who serve better the interests of their parents than their own. Finally, parents can inflict a cost to their children through the use of social institutions (i). Ascending the social hierarchy takes time, and consequently most social institutions (e.g., the church) are controlled by the older generation (Apostolou 2013b). In effect, parents may find themselves in higher positions or they may have relatives and friends in high places in these institutions. Thus, parents have the capacity to use these social structures to impose a cost on their children for deviating from what is considered desirable behavior. For instance, a religious institution may punish a premarital sexual relationship, or the law may prevent a marriage to take place if it does not have the approval of parents. Based on the above analysis, the (d) is decomposed in the following factors:

Theory Biosci.

d ¼ w þ e þ h þ j þ i: When parents inflict a cost to their children in order to align them with their will, they also inflict a cost to themselves as they are genetically related to their daughters and sons; so, the net cost they can inflict on their children is over and above the cost they inflict to themselves. Accordingly, the (d) needs to be adjusted by ( r ) multiplied by the cost inflicted to parents through inflicting a cost to their children (q) so that d ¼ ðw þ e þ h þ j þ iÞ  q  r: Finally, when parents attempt to impose a cost to their children, the latter have a capacity to absorb or resist it. For instance, children who are low in agreeableness and conscientiousness may better resist psychological manipulation; children of considerable social status may be more resistant to the mandates of social institutions; physically stronger children can better resist their parents applying force on them; older children are less dependent on their parents protection and resources; the wealth of parents matters less to children who are wealthy themselves and so on. Accordingly, the (d) should be considered as a net cost, which is the cost that parents can inflict on their children to align them with their will over and above the one the children can absorb. In sum, the total opportunity cost is expressed as: odiverging ¼ ðb  f  c  rÞ  v  d: Converging opportunity cost The genetic relatedness between parents and their children (i.e., r = 0.5) indicates that the two parties, except from diverging, have also converging interests over mate choice; that is, parents and children both agree in the latter getting a mate who is beneficial for them. Parents, by interfering and controlling mate choice, can enable their children to get better mates than they could get for themselves. Thus, if parents do not interfere, they stand to lose, because their children may not get the best possible mates. Therefore, there is an additional opportunity cost accrued to parents by the free mate choice of their children which, in this case, comes from parents and children having converging interests over mating (oconverging). This cost is predicted by the possible risk that children face to make mistakes when they exercise mate choice, which their parents are less likely to make if they were to exercise choice for them (k). For instance, if children are left on their own to exercise mate choice, they may fall prey to individuals who wish to exploit them or harm them. Parents, on the other hand, are less likely to fall prey to such individuals because they are older and more

experienced, and their judgment is not clouded by high levels of libido and romantic love. The (k) is positively predicted by the traits of children which may turn them prone to make unwise mate choices, and become vulnerable to unfit suitors (t). One such trait is experience, which usually correlates positively with age, with older and more experienced children to be less likely to make wrong choices. In this case, then, lack of experience can positively contribute to the (k). Personality traits are also important factors; for instance, a highly conscientious child is less likely to be carried away in a sexual adventure than a low conscientious one. Intelligence is yet another factor, as children of higher intelligence can better protect themselves and make wiser mate choices. In sum, children with specific traits, such as being less experienced, having low conscientiousness and low intelligence, are more likely to make unwise mate choices, which lead to a higher opportunity cost for their parents if they let them to exercise mate choice on their own. In addition, the (k) is positively predicted by children’s mate value (v). This is because children of high mate value are expected to be more frequently the target of intense mating effort than children of low mate value, which may often involve deception and assault. For example, low mate value men who follow a forced-sex mating strategy will target primarily women at the peak of their fertility (see Thornhill and Palmer 2000). Accordingly, parents risk losing more if they allow an attractive than an unattractive daughter to exercise choice on her own. Thus: k = t ? v. Note that the (t) and the (v) are not independent, as certain traits such as age predict both the (t) and the (v); accordingly, this relationship can be studied and modeled better by future work. Moving on, the (k) needs to be adjusted by the impact that a wrong mate choice will have on children and thus, on their parents (n). The (n) is a function of several variables including the presence of sexually transmitted diseases. For instance, the (n) is higher when HIV prevalence is high and lower when HIV prevalence is low. Similarly, the (n) is lower when a woman uses contraceptives than when she does not. The converging opportunity cost is also predicted by the child’s risk to be unable to attract a mate (l). This is costly for parents, who have directed a considerable investment to raise this child to sexual maturity, and they risk forfeiting it in the case that their child is unable to give them grandchildren. This risk is positively predicted by the personality traits of a child that negatively influence success in attracting a mate (s). For instance, children who score high on introversion are more at risk of not being able to attract mates, if they are left on their own. Thus, the higher a child scores in such traits, the higher the value of (s). This risk is also a negative function of a child’s mate value (v): As the

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mate value of a child decreases, the risk of not being able to attract a long-term partner increases. The intervention of parents however, can decrease this risk. For instance, parents may find mates for their children when the latter’s personality traits prevent them from doing so. In addition, parents may interfere, diverting more resources to their children in order to increase their mate value. For example, parents may augment the dowry of a daughter to compensate for a physical deformity that impairs her mating success; by doing so they have more say in the choice of an in-law. Thus: l ¼ s þ 1v. Finally, the converging opportunity cost needs to be multiplied by the adjusted r ð r Þ to get an actual estimate of the cost that parents will suffer if their children make wrong mate choices or are unable to get a mate and procreate. Accordingly: oconverging ¼ ½ðk  nÞ þ l  r: The converging opportunity cost gives an additional incentive to parents to control the mate choices of their children, a control which is also beneficial for the latter. This potential benefit indicates that children may consider a degree of parental intervention and say over their mate choices acceptable or even desirable. Thus, they are also likely to ask the advice of their parents in matters of mate choice or allow them space to intervene, giving in effect space to parents to influence mating decisions. The opportunity cost from converging interests strengthens parental choice as a sexual selection force since even if, in this case, parents attempt to control the mate choices of their children because they want the best for them, when they succeed in doing so they are more likely to get in-laws who best appeal to their preferences. For instance, if their son is constrained by a physical defect in finding a mate, his parents may arrange a marriage for him, paying a higher bridewealth and making compromises on the traits of a prospective daughter-in-law. But, the compromises they are going to make will be in accordance to their own preferences and not to their son’s preferences. What is important in sexual selection is who exercises choice, not why. It has to be said that empirical work is required to model the risks that give rise to the converging opportunity cost (i.e., the k and the l). For instance, the mate value of children affect both risks but differently: As mate value increases, the (k) increases, since children are more likely to become the target of unwanted attention, but, at the same time, the (l) decreases as children become less likely to stay single. The two effects, to some degree, cancel out so, the mate value may not have a considerable quantitative effect in the converging opportunity cost. It is likely to have a qualitative effect though, as when the (k) is high and the (l) is low, parental interference will take the form of guarding, acting aggressively or threatening

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unsuitable mates, while when the (k) is low and the (l) is high, parental interference will take the form of finding mates and/or taking measures to enhance the mate value of their children. Note that the level of converging opportunity cost is not determined by the cost that parents can inflict on their children to align them with their will (d), because children will be willing to surrender some of their freedom to exercise choice to their parents, as doing so will be also beneficial for them. The freedom that children will be willing to surrender would equal the converging opportunity cost; in order to get more of their children’s freedom, and thus, to gain from reducing the diverging opportunity cost, parents will need to inflict a cost to their children (d). Therefore, even if parents can inflict no cost to their children, they will still be influential over mating because their children will be willing to surrender some of their freedom to them. In sum, free mate choice involves an opportunity cost which motivates parents to control the mate choices of their children: m ¼ o where o ¼ odiverging þ oconverging




Substituting the factors influencing the (odiverging) and the (oconverging): m ¼ ½ðb  f  c  rÞ  v  d þ f½ðk  nÞ þ l  rg Substituting the (d): m ¼ fðb  f  c  rÞ  v  ½ðw þ e þ h þ j þ iÞ  q  rÞg þ ðk  n þ lÞ  r Substituting the (k) and the (l): m ¼ fðb  f  c  rÞ  v  ½ðw þ e þ h þ j þ iÞ  ðq  rÞg þ ½ðt þ vÞ
 n þ s þ v1   r This model formalizes and synthesizes previous formulations (i.e., Apostolou 2007, 2010, 2012), while it makes also several expansions and additions. The most important addition, is that it introduces and defines the converging opportunity cost of free mate choice, a cost that has not been considered in previous formulations. Furthermore, in previous formulations, parental control over mating was predicted by the opportunity cost arising from diverging interests plus the ability of parents to place mate choice under their control. In this revision, the capacity of parents to inflict a cost to their children in order to align them with their will (d) is treated as a determinant of the

Theory Biosci.

level of the diverging opportunity cost. Last but not least, this revision incorporates a more comprehensive list of factors that are likely to influence the opportunity cost of free mate choice, and thus, of parental control over mating. The model predicts specific contingencies in parental choice that will be explored next.

Contingencies in parental choice Daughters vs. sons Women divert more parental investment to their children, and so, they become the scarce reproductive resource to which men seek access (Trivers 1972). This asymmetry translates into a daughter being a more valuable reproductive resource than a son (vdaughters [ vsons). Moreover, women usually control less wealth than men (Whyte 1978), which means that daughters are more, and for a longer period, dependent on their parents’ resources than sons. In addition, women are physically weaker than men, which translates into daughters being more dependent on their family’s protection than sons (edaughters [ esons). Furthermore, because daughters are physically weaker than sons, parents can more easily employ physical force to inflict a cost on the former than on the latter (hdaughters [ hsons). Overall, the diverging opportunity cost of free mate choice is higher for daughters than for sons (odiverging_daughters [ odiverging_sons). A sexual adventure can commit a daughter’s parental investment (i.e., pregnancy) to a man who is not necessarily optimal for her. An unwanted pregnancy is not something that can be easily resolved, particularly in a preindustrial context. Also, not only a son does not need to bear the cost of pregnancy, but he can also deny paternity, since in a pre-industrial context this is not something that can be easily proven. Moreover, due to parental uncertainty, men value chastity in a woman as this preference reduces the risk of cuckoldry (Buss 2003). For a similar reason, parents prefer a daughter-in-law who is chaste (Apostolou 2014c); a preference that reduces the risk of diverting resources in grandchildren which are not their own. In addition, due to menopause, the reproductive value of a daughter, after sexual maturity, decreases more rapidly with age than the reproductive value of a son. Thus, erroneous mate choices can have more detrimental consequences for the fitness of daughters than for the fitness of sons (ndaughters [ nsons). Furthermore, because women constitute the scarce reproductive resource, they will be the target of more intense mating effort, and thus, they will face a higher risk of deception and assault by unfit men, who attempt to gain access to their reproductive capacity. Overall, the risk of

erroneous mate choices is higher for daughters than for sons (kdaughters [ ksons). On the other hand, women, by being the scarce reproductive resource, they are less at risk of not being able to attract a mate than men (ldaughters \ lsons). As the (i) and the (k) are higher, but the (l) is lower for daughters than for sons, assuming an equal contribution of each factor, the converging opportunity cost of free mate choice is higher for daughters than for sons (odiverging_daughters [ odiverging_sons). As both diverging and converging opportunity costs are higher for daughters than for sons, parental control over mating would be stronger over daughters than over sons (mdaughters [ msons). Note that this difference reflects the general trend, and does not apply to all situations. It may be the case, for instance, that parents have two children, a young son and an older daughter who has passed the age of menopause and she is financially independent. In this case, the opportunity cost of free mate choice is higher for the son than for the daughter, which predicts a stronger parental control on the mating decisions of the former than of the latter. Mothers vs. fathers The asymmetry in parental investment motivates men not only to compete between them, in being selected by women or by their parents (intersexual selection), but also to fight directly between them (intrasexual selection). The consequence of intrasexual selection forces is for men to be physically stronger than women, to tend to monopolize resources, to dominate the political scene and have exclusive access to weaponry (Puts 2010). Thus, fathers and other male relatives, control more resources than mothers and other female relatives, which in turn can be withheld if their children disobey them (wfathers [ wmothers). One clarification than needs to be made at this point is that although mothers usually divert more parental investment to their children than fathers, this does not mean that they have a stronger say in mating. One reason is that they divert more of what they have, but not more in absolute terms. Since fathers usually control more resources, they can potentially divert more of these to their children. Therefore, children risk losing more if they disobey their fathers than if they disobey their mothers, even if mothers divert more of what they have to them. Another reason is that mothers divert more parental investment to their children when the latter are young (e.g., breastfeeding). Nevertheless, older children are less dependent on the investment of mothers (e.g., they do not breastfeed), and more on the resources of their family in general, which are controlled by the father. For instance, in pastoral societies sexually mature offspring base their

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subsistence on the herding of animals, which belong to the father (e.g., the pastoral Fulbe see Hopen 1958). Furthermore, the defense of the family unit rests on its male members, who are typically physically stronger, control weaponry and have political connections. Therefore, children are more dependent on their fathers than on their mothers for protection (efathers [ emothers). Also, because fathers are physically stronger than mothers, they can inflict a higher cost by using physical force on their children (hfathers [ hmothers). In the same vein, because men have better access to political institutions (Apostolou 2013b), fathers can more readily employ them to inflict a cost to their children than mothers (ifathers [ imothers). Women, on the other hand, by being physically weaker than men, have to rely on psychological manipulation in order to promote their interests (Apostolou 2013a). In consequence, mothers are usually more effective manipulators (Apostolou and Papageorgi 2014), and thus, they can inflict a higher psychological manipulation cost on their children than fathers can (jfathers \ jmothers). In addition, due to internal gestation, mothers are 100 % certain that their children are their own; this is not the case for fathers however, who are less certain about paternity. As a consequence, the adjusted r is lower for fathers than for mothers ( r fathers \ rmothers), which means that the cost that parents suffer from inflicting a cost to their children (q 9 r) is lower for fathers than for mothers. Overall, fathers can inflict a higher cost than mothers on their children to align them with their will (dfathers [ dmothers). A further implication of the paternal uncertainty, is that the costs from making compromises in order to get benefits from traits (c 9 r) is less for fathers than for mothers. In consequence, fathers can potentially gain more from a mating deal than mothers, as they are less constrained by the cost that this may have on their children. Moreover, due to menopause, women conclude their reproductive careers at an earlier age than men. This difference means that the residual reproductive value (i.e., the contribution to the population through future reproduction) is less for older women—to the point of being zero if they have passed the age of menopause— than it is for men of the same age (Apostolou 2014b). As a consequence, a mating deal such as an arranged marriage can be more beneficial for fathers, because it can provide them with resources that can be used for future reproduction. In addition, men have a higher reproductive variance than women, as they are not constrained by their biology in the number of children they can father. Men’s reproductive success is positively related to the resources they control (Buss 2003), with men being able to deploy resources in such a way that enables them to practice polygyny and/or to attract multiple casual mates (Goode 1982). On the other

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hand, because women are constrained by their biology, polyandrous marriage and having multiple casual mates will not increase their reproductive success. Accordingly, a mating deal for their children, which would provide parents with resources, can potentially be more beneficial for a father than for a mother, as the former could use these resources to directly increase his reproductive success— that is, the resources obtained from a mating deal have the potential to increase the direct reproductive success of the father to a considerably greater degree than the direct reproductive success of the mother (ffathers [ fmothers) (Apostolou 2014b). In sum, because fathers can inflict more cost to their children, and because they can convert more of the benefits they extract by controlling mate choice in reproductive success, they have more to lose if they allow their children to exercise mate choice freely (odiverging_fathers [ odiverging_mothers). This argument refers to the typical case, and it does not preclude the possibility that the diverging opportunity cost is higher for a mother than for a father. For instance, in the case of a family where the father is very old but the mother is relatively young, the latter can inflict more cost to children, and she is more likely to convert benefits from a mating deal into a direct reproductive success (i.e., in order to get a desirable husband after her husband dies or a lover) than the former. On the other hand, due to paternal uncertainty, if children make erroneous choices which reduce their fitness, mothers have potentially more to lose because they are more certain that their children are actually their own; consequently: (oconverging_fathers \ oconverging_mothers). When the diverging opportunity cost is higher than the converging opportunity cost, control over mating would be dominated by fathers (mfathers [ mmothers), but when the diverging opportunity cost is lower than the converging one, control over mate choice would be dominated by mothers (mfathers \ mmothers). Which opportunity cost is going to be higher, and thus, who of the parents exercises greater control over mating, depends on the specific conditions prevailing in a given society.

Sexual selection under parental-choice between subsistence types Pre-industrial societies vs. post-industrial societies Post-industrial societies are more technologically advanced than pre-industrial ones and they produce more resources (apre-industrial \ apost-industrial). Accordingly, parents, by controlling their children’s mate choices, can gain more in the former than in the latter (bpre-industrial \ bpost-industrial). In the same vein, parents in post-industrial societies control

Theory Biosci.

more wealth than parents in pre-industrial ones, which means that children have more to lose if they disobey their parents in the former than in the latter (wpre-industrial \ wpostindustrial). On the other hand, technological orientation demands a long period of training before one is able to participate effectively in the labor market; thus, in post-industrial societies children are married at a later age when they are financially independent from their parents. In addition, the presence of social protection systems, such as the police, result into children being less dependent on their families for physical protection. In pre-industrial societies, however, children are married relatively young, since a long period of education or training is not required, and they are, therefore, more dependent on the resources of their parents (Apostolou 2014c). Also, as the social protection systems are not well-developed or inexistent, children have to rely on their parents for physical protection. Accordingly, children are much more dependent on their parents’ resources in pre-industrial than in post-industrial societies (epre-industrial [ epost-industrial). In post-industrial societies, individual rights are well protected and parents are prevented by the rule of law to use physical force on their children. But even if they were allowed to do so, this would not be easy, since children are getting married at an older age, when they are usually physically stronger than their parents. On the other hand, in pre-industrial societies children are married younger, which means that their parents are also younger, and thus better able to use physical force on them. In sum, the capacity of parents to apply physical force on their children is larger in pre-industrial societies than in post-industrial ones (hpre-industrial [ hpost-industrial). In addition, in post-industrial societies social institutions aim predominantly at protecting individual rights, and consequently, the older generation cannot easily employ them to inflict cost on the younger generation. On the other hand, in pre-industrial societies parents can more readily employ institutions, such as the church or the secular law, to impose their will. In effect: (ipre-industrial [ ipost-industrial). Overall, assuming an equal contribution of each factor, parents in pre-industrial societies have a higher capacity to inflict costs on their children in order to align them with their will than parents in post-industrial societies (dpre-industrial [ dpost-industrial). Furthermore, children in post-industrial societies are married at an older age than children in pre-industrial ones, which means that their parents are also older and have a lower reproductive capacity. Therefore, any potential gains from control over mating have a limited potential to be converted into additional offspring in post-industrial than in pre-industrial societies. This capacity is further reduced by the fact that polygyny is not allowed in the vast majority

of post-industrial societies. This is not, however, the case for pre-industrial societies, where polygyny is usually allowed (Murdock 1967) and children are married young, and have relatively young parents who can convert benefits from a marriage deal into additional children. Therefore: (fpre-industrial [ fpost-industrial). In addition, in post-industrial societies children are married at a later age so, they are more experienced, which means that they can make better choices; accordingly: (kpreindustrial [ kpost-industrial). Also, the widespread use of contraceptives and the after day pill, along with the effective treatment of most sexually transmitted diseases, turn erroneous mate choices less consequential in post-industrial societies (npre-industrial [ npost-industrial). In sum, the converging opportunity cost is lower in a post-industrial context (oconverging_pre-industrial [ oconverging_post-industrial). Overall, the opportunity cost of free mate choice is higher in pre-industrial than in post-industrial societies (opre-industrial [ opost-industrial), which predicts that parental control over mating is higher in the former than in the latter (mpreindustrial [ mpost-industrial). In pre-industrial societies wealth is predominantly controlled by the male members of a society, while in postindustrial societies it is more equally distributed across the sexes (Apostolou 2014c). Furthermore, men in pre-industrial societies are less constrained in employing their physical strength in controlling their children; thus: (dfathers_pre-industrial [ dfathers_post-industrial) [ (dmothers_pre-industrial [ dmothers_post-industrial). Also, fathers can benefit more by a marriage arrangement (b) in a pre-industrial context as the extracted wealth goes under their control (e.g., the bridewealth see Goody and Tambiah 1973). As discussed above, in pre-industrial societies, as opposed to post-industrial ones, polygyny is permitted and parents are relative young when their children get married. However, this situation favors predominantly men who have a higher residual reproductive value and a higher reproductive variance than women. Accordingly: (ffathers_pre-industrial [ ffathers_post-industrial) [ (fmothers_pre-industrial [ fmothers_post-industrial). Overall, the diverging opportunity cost for male parents is higher in pre-industrial than in post-industrial societies, which predicts that male parental influence will be higher in the former than in the latter. The diverging opportunity cost of free mate choice is likely to decrease dramatically in post-industrial societies as parents can inflict limited cost on their children to align them with their will, with the converging opportunity cost becoming the primary component of the overall opportunity cost. Nevertheless, due to paternal uncertainty, the converging opportunity cost is higher for mothers than for fathers (oconverging_fathers_post-industrial \ oconverging_mothers_post-industrial), which predicts that in post-industrial societies parental influence will come predominantly from mothers.

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Theory Biosci.

In sum, it is predicted that parental control over mating will be stronger in pre-industrial than in post-industrial societies, and male parents will be more dominant in controlling mating in the former than in the latter. Consistent with this prediction, in all post-industrial societies marriage is, typically, based on free-choice, but parents exercise an influence on their children’s mate choices, primarily through the use of manipulation (Apostolou 2013a, b). In particular, parents employ at least 12 different tactics to manipulate their children into accepting a desirable relationship or breaking an undesirable one. They also employ at least four tactics on their children’s partners in order to make them stay in the relationship if they consider them desirable, or to go away if they consider them undesirable (Apostolou 2013a, b). In addition, mothers indicated a higher willingness to use manipulation than fathers. Furthermore, another study found that mothers indicated a greater interest in influencing the mate choices of their children (Apostolou 2011b see also Apostolou and Papageorgi 2014). The effectiveness of parental tactics in mate choice manipulation remains to be estimated; still, it is clear that this influence is weaker in post-industrial societies than it is in pre-industrial societies, where parents are able to control mate choice directly. More specifically, one study collected evidence for a sample of 190 contemporary foraging societies and analyzed their mating patterns (Apostolou 2007). It was found that the most common mode of longterm mating, in approximately 70 % of the cases, is arranged marriage, where parents choose spouses for their children. In fewer than 5 % of cases is the primary mode of marriage courtship, where children choose their own spouses with little input from their parents. Another study that employed data on mating patterns from the standard cross-cultural sample consisting of 186 hunting and gathering and agropastoral societies, found similar results; that is, arranged marriage was the most frequent form of longterm mating (Apostolou 2010). Both studies also found that marriage arrangements are dominated by male parents, while parental control is directed predominantly on the female offspring. Hunting and gathering vs. agropastoral societies Agropastoral societies are more technologically advanced than foraging ones, and consequently, they produce more material wealth (aagropastoral [ aforaging). Accordingly, individuals in the former are likely to have more wealth at their disposal than individuals in the latter. For instance, individuals in agropastoral societies are likely to own land, animals, food surpluses, and money. On the other hand, individuals in hunting and gathering societies are likely to own artifacts, weapons and household objects. This means

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that parents can gain more resources from controlling mating in agropastoral than in foraging societies (bagropastoral [ bforaging). In the same vein, parents are likely to control more wealth in agropastoral than in foraging societies, which means that children have more to lose if they disobey their parents in the former than in the latter societies (wagropastoral [ wforaging). Furthermore, in hunting and gathering societies, individuals can rely on their own hunting or gathering effort in order to sustain themselves. In agropastoral societies, on the other hand, they need to rely predominantly on the cultivation of land or the herding of animals, which are owned by their parents. In effect, individuals are more dependent on their parents’ resources in the former than in the latter societies (eagropastoral [ eforaging). Agropastoral societies are larger, as they produce more resources that allow the maintenance of a bigger population, and they are more complex, as there is a higher degree of specialization. Thus, powerful social institutions, such as the law and the church, are required in order to make the functioning of these societies possible. For example, a sophisticated legal system and a refined theological dogma are only found in agropastoral societies and not in hunting and gathering ones (Apostolou 2013b). These social institutions are controlled by older generations, who have shaped them in such a way so as to regulate mate choice (Apostolou 2013b). Consequently, parents can impose a higher cost on their children in agropastoral societies through manipulating social institutions (iagropastoral [ iforaging). In sum, parents can inflict a higher cost on their children in agropastoral than in foraging societies (dagropastoral [ dforaging), which translates into the opportunity cost of free mate choice being higher in agropastoral societies than in foraging ones (oagropastoral [ oforaging), predicting, in turn, a stronger control over mating in the former than in the latter (magropastoral [ mforaging). Due to intersexual competition, men usually monopolize resources (Buss 2003); thus, when more resources are available, men have control over more of them. Accordingly, in agropastoral societies which produce more wealth, children have more to lose if they disobey their fathers than their mothers, than in hunting and gathering societies: (wfathers_agropastoral [ wfathers_foraging) [ (wmothers_agropastoral [ wmothers_foraging). In addition, the presence of more resources increases the incentive of rival groups to monopolize them, which means that there is a higher need to defend the group and its members. Accordingly, children are more dependent on their male parents’ protection in agropastoral than in foraging societies (efathers_agropastoral [ efathers_foraging). The higher amount of produced resources results also in greater emphasis to be placed on military preparation, which is the realm of men (Apostolou 2010). As a

Theory Biosci.

consequence, more sophisticated and lethal weapons are produced, to make the defense of resources more effective, and/or to get more effectively the resources of other groups. To these weapons, men usually have an exclusive access, which amplifies their ability to impose their will on their children by force (hfathers_agropastoral [ hfathers_foraging). Finally, social institutions are controlled predominantly by older men, and since these institutions are more likely to be present in agropastoral than in foraging societies (Apostolou 2013b), the male parents’ capacity to use them for inflicting costs on their children is higher in the former than in the latter (ifathers_agropastoral [ ifathers_foraging). Overall, fathers have a greater capacity to inflict a cost to their children in agropastoral than in foraging societies (dfathers_agropastoral [ dfathers_foraging), which means that the diverging opportunity cost of free mate choice is higher for them in the former than in the latter societies (odiverging_fathers_agropastoral [ odiverging_fathers_foraging). Therefore, male parental control over mating is predicted to be stronger in agropastoral than in foraging societies. Moreover, in comparison to foraging societies, in agropastoral ones, inheritance rights are more likely to favor male children (Apostolou 2014c). Thus, male children have more to lose if they disobey their parents in the former than in the latter. In effect, male parents can inflict a higher cost on their male children in agropastoral than in foraging societies. Furthermore, because men control more resources in agropastoral societies, and they play a more vital role in the defense of resources, they have the capacity to contribute more parental investment to their children. Accordingly, the mate value of sons is higher in agropastoral than in foraging societies (vsons_agropastoral [ vsons_foraging), which means that parents have more to lose if they allow their sons to exercise mate choice freely in the former than in the latter societies. Overall, the opportunity cost of male offspring’s free mate choice is higher in agropastoral than in foraging societies (osons_agropastoral [ osons_foraging), which predicts that parental control over mating would be stronger over male offspring in agropastoral than in foraging societies. The predictions of the model are consistent with the anthropological record. In particular, based on evidence from a cross-cultural sample, one study made comparisons in the patterns of mating between societies which base their subsistence on hunting and gathering and those which base their subsistence on agriculture and animal husbandry (Apostolou 2010). The analysis indicated that parental choice is more dominant in agropastoral than in foraging societies, as arranged marriage is more frequently found in the former than in the latter. Furthermore, in agricultural and pastoral societies male parents and relatives have more decision-making power over marriage arrangements than female ones. Actually, there

was not even a single case reported where female relatives dominated marriage arrangements. In addition, increased parental control came predominantly from controlling the mate choices of sons, as marriage was more frequently arranged for sons in agropastoral than in foraging societies.

Discussion This paper constitutes the first attempt to provide a formal model of parental choice, while it incorporates a number of expansions and additions. The most important expansion constitutes the introduction of the converging opportunity cost, while the most important revision constitutes the treatment of the cost that parents can inflict to their children as a part of the opportunity cost of free mate-choice. The proposed model has been applied in accounting for part of the variation in the mating patterns across different society types. Although the predictions derived from the model fit the observed cross-cultural patters, it has to be said that there are many different and diverse factors which are likely to have an impact on mate choice in a given society. Thus, it is difficult or impossible for a single attempt to account for the cross-cultural variation in mating patterns. Accordingly, the proposed model should not be seen as adequate for accounting for such variation, but instead, as useful for providing insights on how matting patterns are likely to change across different society types. This revision incorporates a more comprehensive list of factors that are likely to influence the opportunity cost of free mate choice, and thus, the strength of parental control over mating. One danger of this endeavor is that the model may appear to lack parsimony. Yet, although in this formulation there are many factors that determine the opportunity cost of free mate choice, the conception of the model remains simple: The genetic relatedness between parents and children gives rise to an opportunity cost of free mate choice which makes it beneficial for the former to control or influence the mate choices of the latter. The level of this opportunity cost is determined by several factors that have been identified here. It needs to be said, however, that more theoretical and empirical work is required in order to model better the relationship of these factors (i.e., to estimate their weights) with the opportunity cost of free mate choice. For instance, some factors may weight much more and some much less in affecting the opportunity cost. In addition, this paper does not claim to have identified all the factors that predict the opportunity cost of free mate choice, and future work can come up with a more inclusive list.

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Theory Biosci.

Furthermore, certain variables, such as the mate value, are entered in different arguments of the equation, which means that they can be factored out to produce a more frugal expression of the model. This is not done here primarily for two reasons: First, the purpose of this paper is to model the opportunity cost of free mate choice and examine how individual factors affect its different arguments. Thus, producing a more elegant expression may turn less apparent how the variables contribute to each argument. Second, this endeavor is premature at this stage as the exact relationship between the different variables remains to be estimated. One substantial revision put forward in this paper, is that parental control over mating is positively predicted by converging opportunity cost. It has to be said, however, that this converging opportunity cost is expected to be smaller than the diverging opportunity cost. In particular, due to the importance of mate choice, evolutionary forces should have forged mechanisms, such as mate preferences, to prevent individuals from making erroneous mate choices (Buss 2003) and mechanisms, such as libido and romantic love, to prevent them from staying single. In consequence, the converging opportunity cost should, on average, be small, which in turn means that the diverging opportunity cost is the primary component of opportunity cost, and thus a more important reason for controlling mating. In the current formulation, the converging opportunity cost is multiplied by the adjusted genetic relatedness, which makes it smaller than the diverging opportunity cost, but future studies need to attempt to estimate and model more accurately the contribution of the converging opportunity cost to the opportunity cost of free mate choice. It is important to say that sexual selection is a complex process, and the present paper does not capture all of its dynamics for the human species. Actually, this is not the purpose of this work, which aims to study one specific sexual selection force, namely parental choice. There have been some attempts to provide overviews of sexual selection theory (e.g., Kuijper et al. 2012), which however, need to be expanded and revised, by taking into consideration parental choice, a selection force which is unique in the human species. To conclude, parental choice is likely to have played an important role in shaping the human mind. To study how this might have come to be the case, it is necessary to identify and model the factors that predict parental control over mating. This paper has provided a first attempt to do so, and future studies need to expand and refine the proposed model. Acknowledgments The author would like to thank Georgia Kapitsaki and two anonymous reviewers for their constructive feedback which enabled the improvement of this work.

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Appendix: Factors (a) The level of available resources in a given context. (b) The benefit parents receive in terms of desirable traits when they control mate choice. (c) The cost that children suffer when their parents control their mating decisions. (d) Parents’ capacity to inflict a cost to their children in order to align them with their will. (e) The degree of children’s dependence on their parents’ resources. (f) The potential of (b) to be converted into fitness benefits. (h) The cost that parents can impose on their children by applying physical force. (i) The cost that parents can impose on their children through social institutions. (j) The psychological cost that parents can impose on their children. (k) The risk that children face to make mistakes when they exercise mate choice. (l) The risk that a child faces of not being able to attract a mate. (m) The strength of parental control over mating. (n) The impact that a wrong mate choice will have on children. (oconverging) The opportunity cost of free mate choice coming from converging genetic interests. (odiverging) The opportunity cost of free mate choice coming from diverging genetic interests. (p) The probability that one is actually an individual’s genetic relative. (q) The cost parents suffer when they inflict a cost to their children. (r) The degree of genetic relatedness. ( r ) The degree of genetic relatedness (r) multiplied by the probability (p) that one is actually an individual’s genetic relative. (s) The personality traits of a child that negatively influence success in attracting a mate. (t) The traits of children which may turn them prone in making unwise mate choices. (v) An individual’s mate value. (w) Parental resources which are diverted to children in the form of parental investment. (x) The personality traits of a child that negatively influence success in attracting a mate.

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