Journal of Marital and Family Therapy doi: 10.1111/jmft.12089 October 2014, Vol. 40, No. 4, 430–441

CONDITIONAL INFERENCE TREES: A METHOD FOR PREDICTING INTIMATE PARTNER VIOLENCE Katie Lee Salis Stony Brook University

S€ oren Kliem Criminological Research Institute of Lower Saxony

K. Daniel O’Leary Stony Brook University

A number of different methodologies have been employed to investigate the complex relationship between psychological and physical aggression. Herein, a method of unbiased recursive partitioning (conditional inference trees) was applied to a longitudinal sample to identify cutoffs of psychological aggression at baseline that differentiate between individuals who do and do not perpetrate physical aggression at follow-up. The algorithm categorized men into low- and high-risk groups, and women into mild-, moderate-, or high-risk categories of perpetration. Couples responded anonymously to a self-report measure of psychological and physical aggression (CTS2) at baseline and a 12-month follow-up. Sensitivity analyses for predicting physical aggression reached as high as 59% for women and 60% for men. The perpetration of psychological aggression is one of the most robust risk factors for the perpetration of physical aggression (Schumacher, Feldbau-Kohn, Smith Slep, & Heyman, 2001; Stith, Smith, Penn, Ward, & Tritt, 2004). Psychological aggression herein is defined as “the use of verbal and nonverbal acts which symbollically hurt the other, or the use of threat to hurt the other” (Straus, 1979). These acts may include “humiliating the victim, controlling what the victim can and cannot do, withholding information from the victim, deliberately doing something to make the victim feel diminished or embarrassed, isolating the victim from friends and family, and denying the victim access to money or other basic resources” (Saltzman, Fanslow, McMahon, & Shelley, 2002). Stith et al. (2004) noted that emotionally abusing a partner has one of the largest effect sizes, only rivaled by forcing a partner to have sex, as a risk factor for physical aggression. Further, there is consistent evidence that one’s own psychological aggression, regardless of gender, is predictive of later and persisting perpetration of physical aggression (Aldarondo & Sugarman, 1996; Murphy & O’Leary, 1989; O’Leary, Malone, & Tyree, 1994; Schumacher & Leonard, 2005). In clinical samples, researchers regularly report high prevalence rates of psychological aggression, some as high as 95% for both men and women (Simpson & Christensen, 2005). Even with more conservative estimates, prevalence rates for psychological aggression in representative samples are between 75–81% (Bell & Naugle, 2007; Carney & Barner, 2012; Stets, 1990). Thus, psychological aggression is probable in both clinical and representative samples. Though psychological aggression is widespread, the rates of physical aggression are much lower. Clinical samples of couples seeking marital therapy endorse perpetration of physical aggression at rates between 36% and 58% (Jose & O’Leary, 2009). Representative samples tend to physically aggress at lower, albeit still troubling, yearly rates between 9% and 12% (Schafer, Caetano, & Clark, 1998; Straus & Gelles, 1990). It is important to note that for those couples who do seek therapy, a wealth of evidence suggests that physical aggression is grossly underreported, suggesting that prevalence rates as reported may be deceivingly low (Bograd & Mederos, 1999; Doherty &

Katie Lee Salis, MA, K. Daniel O’Leary, PhD, Stony Brook University; S€ oren Kliem, PhD, Criminological Research Institute of Lower Saxony. Address correspondence to Katie Lee Salis, Department of Psychology, State University of New York at Stony Brook, Stony Brook, New York, 11794; E-mail: [email protected]

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Simmons, 1996; Ehrensaft & Vivian, 1996; Froerer, Lucas & Brown, 2012). Even with consideration to the issue of underreporting, it is clear that many more individuals are psychologically aggressive than physically aggressive. There is inherent value in the identification of psychological aggression as a risk factor for the perpetration of physical aggression. However, the predictive value of psychological aggression diminishes if it is highly pervasive in those who are both physically aggressive, and in those who are not physically aggressive. Most couples report psychological aggression, but many fewer report physical aggression. Further, physical aggression in the absence of psychological aggression occurs at extremely low rates (between 0.2 and 0.4% in representative samples; Stets, 1990). Therefore, it stands to reason that there is a certain level of psychological aggression at which physical aggression becomes more likely. Thus, a key question for a practitioner is: at what level of psychological aggression will physical aggression likely occur? From a theoretical standpoint, the connection between physical and psychological aggression is complicated, and numerous methods have been used to further understand the relationship between these two variables. Previous research has delved into the complex association between physical and psychological aggression by employing at least five distinct methods, which are explained herein. The first and one of the most common methodologies employed to study the relationship between physical and psychological aggression is the simple correlation. Results of this work have provided us with consistent evidence that physical and psychological aggression are positively correlated with one another in large and representative samples rs=.56–.65 (Hines & Saudino, 2003; O’Leary, Smith Slep, & O’Leary, 2007). As the level of psychological aggression increases, the level of physical aggression increases, and vice versa. The second method by which previous researchers have investigated the relationship between physical and psychological aggression is that of factor analysis (Marshall, 1992; Murphy & Hoover, 1999; Rodenburg & Fantuzzo, 1993). The Conflict Tactics Scale-II and the original CTS (Straus, 1979; Straus, Hamby, Boney-McCoy, & Sugarman, 1996) are the well-validated measures of intimate partner aggression and have a number of subscales designed to measure disparate types of aggression. Among others, the Revised Conflict Tactics Scale (CTS2) includes both separate physical and psychological aggression subscales. Factor analytic studies have been designed to assess whether these factors (psychological and physical aggression) are independent or overlapping, and to what extent one can develop scales that can be used as independent measures. Overall, studies conclude that the psychological and physical factors and scales are independent measures, but that for women, the more severe psychological aggression items may sometimes load on the physical aggression factor (Barling, O’Leary, Jouriles, Vivian, & MacEwen, 1987; Newton, Connelly, & Landsverk, 2001). Among many notable findings using factor analytic methodology, Hamby and Sugarman (1999) found that there are two distinct factors of psychological aggression (minor and severe) and that severe psychological aggression is more closely related to physical aggression than is mild psychological aggression. Third, predictors and risk factors for psychological and physical aggression have been analyzed to determine whether both types of aggression are caused by the same underlying factors, or whether they are two separate phenomena influenced by disparate risk and predictor variables. Using data from the National Family Violence Re-Survey, Stets (1990) found that regardless of gender, factors including age, race, problematic drinking, experiencing aggression when young, approving of aggression, and perpetrating verbal aggression outside of the family, led to perpetration of both verbal and physical aggression inside of the family as an adult. Most of these factors lead nonaggressive individuals to become psychologically aggressive and lead psychologically aggressive individuals to become physically aggressive. However, certain factors uniquely influenced the progression from verbal to physical aggression (race, witnessing aggression in childhood, being female and low income, and being male and approving of aggression), leading Stets to conclude that physical and psychological aggression are two separate phenomena (with differential predictors) and that the progression from psychological to physical aggression is a two-step process. Further, research on typologies of violence suggests that an important risk factor for psychological aggression leading to physical aggression may be the type of aggression perpetrated, or the October 2014

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strata of aggression in which the individual resides (i.e., situational violence vs. intimate terrorism; Johnson, 2010). Although the factors described above do not explicate the process by which psychological aggression becomes physical aggression, research on typologies might suggest that those who inflict significant emotional pain on their victim (severe psychological aggression) are more likely to inflict physical pain (physical aggression) and that these individuals are intimate terrorists and qualitatively different from those who perpetrate mild or moderate aggression in different contexts (situational violence). In a fourth method and a follow-up to the previously mentioned study by Stets (1990), researchers have investigated the relationship between physical and psychological aggression through the lens of behavioral genetics to address common and unique contributors to both psychological and physical aggression. Saudino and Hines (2007) found that genetic influences accounted for 38% of the correlation between psychological and physical aggression, while the rest can be explained by non-shared environmental influences. In other words, psychological and physical aggressions are influenced by the same genetic factors, but the difference between the expression of the two types of aggressions is largely explained by unique environmental influences. According to these models, on average, we may be able to predict that people are predisposed to become aggressive based on their shared genetic factors, but that it is the non-shared environmental factors predict whether one becomes physically or just psychologically aggressive. A fifth method of evaluating the relationship between physical and psychological aggression is that of trying to determine directionality through longitudinal investigations. There is evidence from such studies that perpetration of psychological aggression predates and leads to the perpetration of physical aggression in the future (Leonard & Senchak, 1996; Murphy & O’Leary, 1989; Schumacher & Leonard, 2005). The approach herein examines the issue of directionality in a specific and heretofore unexamined manner. Our previous work (Salis, Salwen, & O’Leary, 2014) examined the likelihood of becoming physically aggressive across time given various percentiles of psychological aggression at baseline. More specifically, we split up men and women by their decile of psychological aggression relative to the rest of the sample (creating 9 categories of subjects; 0–9th percentile, 10–19th percentile and so on). We then analyzed the frequency of physical aggression perpetration in each decile both concurrently and 1 year later. This study concluded that men and women who fell in the 60th percentile of psychological aggression or higher were more likely to become physically aggressive than those who fell below the 60th percentile in psychological aggression. There was a limited relationship between the lower levels of psychological aggression and physical aggression, but as the level of psychological aggression increased, so did the likelihood of physical aggression. As a follow-up to our previous work, the current study further examines cutoffs of psychological aggression that are predictive of physical aggression. While Salis et al. were (2014) able to identify cursory cutoffs of psychological aggression at which physical aggression would become more likely 1 year later, the methodology of analyses was more descriptive than inferential (percentiles and frequencies). For a practicing clinician, the ability to predict which clients may be high risk for becoming physically aggressive, versus those who are at a lower risk is invaluable. Although the previous study identified the aforementioned cutoffs, it was limited with respect to the classification of participants into clinically relevant risk categories based on their level of initial psychological aggression. Herein, we seek to build on the previous work by using more advanced and novel statistical methods on the longitudinal sample to both replicate our previous basic findings, and to create an assessment tool with which practitioners may be able to place clients into data-driven risk categories for the perpetration of future physical aggression. The current study uses self-report scores on the Revised Conflict Tactics Scale (Straus et al., 1996) at a baseline assessment to predict the likelihood of aggression 12 months later. Analyses include a form of non-parametric, conditional decision trees (Hothorn, Hornik, & Zeileis, 2006). Through this algorithm, initially, a cutoff is chosen that maximally differentiates between the physically aggressive and nonaggressive groups. Then, other thresholds are chosen recursively until the algorithm finds no additional cutoffs. These decision trees enabled us to identify cutoff points of scores on psychological aggression at which physical aggression becomes more likely. Based on initial levels of psychological aggression, these decision trees enable both clinicians and researchers to

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begin to categorize clients and participants by their risk level for becoming physically aggressive in the future.

METHOD Participants We selected a subset of 299 women and 268 men from a larger sample of 453 couples from Suffolk County, New York who were previously recruited for a study on multivariate predictors of physical aggression (O’Leary et al., 2007). This smaller subset was selected for the current study because all participants in this group participated in both waves of the study (baseline and 12 months postmarriage). No significant differences in demographics or in levels of physical aggression were found between this smaller subset of completers and those who dropped-out of the study. This sample included couples who were living together for at least 1 year and were parenting a biological child (of at least one of the parents). 83.5% of men and 83.6% of women reported that they were Caucasian with mean educational levels of 14.3 and 14.4, respectively. Couples were representative in age of the population with young children in the United States, as compared with the 2000 census data (M = 37.6, SD = 5.9 for men, and M = 35.4, SD = 4.9 for women). Measures Psychological and Physical Aggression. The Revised Conflict Tactics Scale (CTS2) is a selfreport scale which measures physical and psychological aggression between romantic partners over the past 1 year period (Straus et al., 1996). It is one of the most widely used tools in the field of intimate partner violence and this revised scale addresses a number of issues with the original CTS, including the addition and revision of several items and the formulation of additional subscales. Although the Revised Conflict Tactics Scale includes an answer choice “This did not happen in the past year, but it has happened before,” we modified our version and did not include this answer choice. To compute scores, we summed the self-report responses on all of the psychological and physical aggression items that were self-reported in the past year, respectively, and calculated the means. See Table 1 for both mean and standard deviation scores for physical and psychological aggression at both time points for men and women. It is important to note that “responses” does not refer to the actual number of times that the event occurred, but to the corresponding numbers from the 7-point scale (0–6). Reliability was high for the perpetration of psychological aggression for men and women (a = .89 and a = .91, respectively), as well as for the perpetration of physical aggression for men and women (a = .79 and a = .82, respectively).

Table 1 Means and Standard Deviation Scores on the CTS2 for Psychological and Physical Aggression Psychological Aggression

Time 1: Baseline Women Men Time 2: Follow-Up Women Men

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Physical Aggression

M

SD

M

SD

1.2 0.99

0.77 0.76

0.12 0.07

0.28 0.21

0.95 0.85

0.74 0.84

0.05 0.05

0.21 0.16

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Procedure Couples came to the lab and completed the CTS2 at 2 different assessment periods; they were assessed at Time 1 (baseline), and then at Time 2 (follow-up 12 months later) along with participating in physiological and observational laboratory tasks. During each assessment period, the husband and wife either independently filled out the CTS2 in a single office with research assistants ensuring confidentiality, or completed the assessments in two separate rooms. All analyses for the current study are derived from the CTS2 during these two assessments.

RESULTS Analyses To understand the predictive relationship between psychological and physical aggression, we conducted nonparametric conditional inference trees using the “Ctree” function in the add-on “Party” package in R (Hothorn et al., 2006). This method creates groups that are maximally different from each other based on the probability of future physical aggression. These inference trees first identify a binary split (or cutoff) in the predictor variable (psychological aggression), creating 2 nodes (subgroups). Above the cutoff, the regression relationship is stronger with the outcome variable (physical aggression) and below, the relationship is significantly less strong. Following the initial binary split, the process is recursively repeated, creating multiple cutoff points, or in our case, levels of risk for future physical aggression (Hothorn et al., 2006; Strobl, Malley, & Tutz, 2009). Conditional inference trees are unique from and, in certain ways, superior to traditional linear regression or stepwise regression methods because they do not rely on underlying assumptions of linearity. Further, this type of prediction and classification method addresses a previous concern of overfitting the sample. “Overfitting” refers to the issue of a statistical model fitting too closely to random error or minor fluctuations in the data, thereby functioning poorly as a predictive tool. This issue is often a byproduct of relying on a complex statistical model (Dietterich, 1995). Another commonly cited issue with other types of decision trees is the bias in selecting variables with more natural cut-points or missing data (Shih & Tsai, 2004). Both issues are addressed and remedied while using the current method by using p values as stopping and selection criteria rather than relying on post hoc pruning methods (Hothorn et al., 2006; Strobl et al., 2009). CutOffs for Predicting Future Physical Aggression in Women We present cutoff points for baseline scores on the psychological aggression questions of the CTS2 at which it is significantly more likely that women in our sample will become physically aggressive 12 months later. The algorithm identified the first statistical cutoff at 2.75. In other words, women who scored above 2.75 on psychological aggression at baseline were significantly more likely (p < .001) to become physically aggressive 1 year later than those who scored below the cutoff. When predicting physical aggression 12 months later, women who scored above 2.75 are categorized high risk, above 1.63 until 2.75 are moderate risk, and those who scored below a 1.63 are considered low risk. See Figure 1 for graphic illustrations of the cutoffs in conditional decision trees for women. CutOffs for Predicting Future Physical Aggression in Men Results of the algorithm applied to men categorized them into two categories: low and high risk. For predicting physical aggression at a 12-month follow-up, men who scored above 1.4 (p < .001) were categorized as high risk, and men who scored at or below 1.4 were categorized as low risk. Although the recursive algorithm searched for further cutoffs for both samples of men, there were no further nodes in the decision trees for the prediction of physical aggression in men at the 12-month follow-up; unlike the women in our sample, men could not be neatly grouped into three different risk categories. Instead, there were only two categories: high risk and low risk (See Figure 2).

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Figure 1. Conditional inference tree predicting women’s physical aggression at 12 months.

Figure 2. Conditional inference tree predicting men’s physical aggression at 12 months Sensitivity and Specificity for the Prediction of Physical Aggression Although conditional inference trees provide a clinician or researcher with data-driven cutoffs for identifying individuals who are more or less likely to become physically aggressive in the future, to give these cutoffs more meaning for use in clinical practice, we have provided sensitivity, specificity, and predictive value scores for the different cutoffs identified above (Table 2). Sensitivity is defined as the ability of the cutoff to correctly identify individuals who will become physically aggressive at follow-up. In other words, sensitivity is the percentage of aggressive individuals who were correctly classified as aggressive. Therefore, if sensitivity is high, those above this cutoff are highly likely to commit violence, and this cutoff represents strong clinical criterion and perhaps contraindication to conjoint couple therapy. A cutoff with lower sensitivity, on the other hand, indicates that those who score above this cutoff may or may not commit violence. If sensitivity is lower, assessment is particularly important. Specificity is the ability of the cutoff to correctly identify the individuals who will not become physically aggressive at follow-up. In other words, specificity is the percentage of nonaggressive October 2014

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Table 2 Sensitivity and Specificity for Future Physical Aggression

Predicting men’s physical aggression postmarriage (12 months) 12 months ≥1.38 (high risk) Predicting women’s physical aggression postmarriage (12 months) 12 months ≥2.75 (high risk) 12 months ≥1.5 (moderate risk)

Sensitivity

Specificity

PPV

NPV

0.60

0.80

.38

.91

0.14 0.59

0.99 0.78

.70 .34

.86 .91

PPV = Positive Predictive Value; NPV = Negative Predictive Value. individuals who were correctly classified as nonaggressive. Therefore, if specificity is high, it is highly likely that individuals below this cutoff are truly not aggressive. Low specificity, however, indicates that it is possible that many individuals who were classified as low risk for aggression will actually perpetrate aggression in the future. Although sensitivity and specificity are measures of the accuracy of the test and do not take rates of aggression into account, positive and negative predictive value are measures of accuracy that are impacted by prevalence of aggression in this sample. For a clinician, an important statistic is the positive and negative predictive value to answer the question: among the people who are classified as high risk, what percentage of them will actually become aggressive? Positive predictive value is the percentage of those categorized as aggressive who are, indeed, aggressive (true positives), while negative predictive value is the percentage of those categorized as non-aggressive who are, indeed, not aggressive (true negatives.) These individuals scored below the cutoff for initial psychological aggression, and as predicted, did not subsequently become physically aggressive. It has been acknowledged that there is no “gold standard” assessment tool for the prediction of violence (Rabin, Jennings, Campbell, & Bair-Merritt, 2009). Although the best overall method of prediction is that which maximizes both sensitivity and specificity, practitioners are often obligated to make decisions that favor one or the other (Mossman, 1994; Roehl, O’Sullivan, Webster, & Campbell, 2005). As described in Mossman (1994), depending on the parameters set by the hospital and the beliefs of the particular clinicians, the consequences of wrongfully hospitalizing a nonviolent individual either outweigh or pale in comparison to those of releasing an individual who may become violent. In our sample, sensitivity and specificity for men are 60% and 80%. Positive and negative predictive values are 38% and 90%. For predicting women’s aggression at follow-up, highest sensitivity and specificity are 59% and 78%. Positive and negative predictive values are 34% and 90%. Table 2 shows sensitivity, specificity, and positive and negative predictive value of each cutoff point for women and men.

DISCUSSION Overall, results suggest that individuals at certain higher levels of psychological aggression at a baseline assessment are more likely to become physically aggressive 12 months later. We used the psychological aggression items on the Revised Conflict Tactics Scale as a screening measure to predict instances of physical aggression in the future by using specific cutoff scores for psychological aggression. Using a recursive partitioning method (decision trees), our sample of men was split into two risk categories (high and low), while our sample of women was split into three categories (high, moderate, and low). By simply measuring one’s level of psychological aggression at baseline, it is possible to categorize and subsequently predict instances of physical aggression into the future with varying levels of sensitivity and specificity.

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Gender Differences A notable gender difference was found for cutoff points for psychological aggression. When comparing the first optimal cutoff points for predicting physical aggression, women had higher cutoffs on psychological aggression than did men. In other words, women and men at the same level of psychological aggression at baseline are not equally likely to become physically aggressive 12 months later (e.g., a woman who scores a 2.4 on initial psychological aggression is considered moderate risk for physical aggression perpetration, while a man who scores a 2.4 is considered high risk.) Women must report higher scores of psychological aggression than men before they are at significant risk to become physically aggressive in the future. To explain these results, one may refer to the literature on gender differences in psychological aggression. In the current sample and others, women are often more psychologically aggressive than men (Hines & Saudino, 2003; O’Leary, Smith Slep, Avery-Leaf, & Cascardi, 2008). Although this may be true, here, and in previous research, women’s level of psychological aggression needs to be higher than that of men to longitudinally predict physical aggression (Salis et al., 2014). Alternately stated, men will begin to engage in physical aggression at lower levels of psychological aggression. Men who become physically aggressive are psychologically aggressive at lower levels than women at all time points, which may lead one to hypothesize that those men who are psychologically aggressive may have a lower “burn-point” or “shorter fuse” than women. It is notable that this interpretation does not consider the level of physical aggression (how many items or how severe); rather, it considers the dichotomous endorsement or nonendorsement of the use of any physical aggression toward one’s partner. Men, more so than women, move from perpetrating a relatively low level of psychological aggression to perpetrating physical aggression. A possible concern with this type of interpretation is whether men are endorsing fewer, but more severe psychological aggression items, discrediting the “shorter fuse” argument. This, however, is not the case; results showed that there were no gender differences in the endorsement of severe psychological aggression items. Further, it is possible that this gender difference may be explicated by a gender-role socialization difference between men and women, beginning at an early age. Specifically, research indicates that mothers react differently to girls’ displays of aggression than to boys’ (Hay et al., 2011; Kingsbury & Coplan, 2012). If physical aggression is less discouraged or punished in boys from a young age, it is likely that the transition from psychological to physical aggression is less inhibited, partly explaining this notable gender difference. Clinical Implications Our work has a number of direct implications and applications for the field of IPV. For practitioners of couple therapy, there is potential clinical utility for these cutoff scores and risk categories. In the past, it had been argued that family and marital therapists were not well appointed to deal with the issues surrounding physical aggression in couples. Researchers had argued that therapists minimize the seriousness of the physical aggression by emphasizing a shared responsibility of both partners, or by simply failing to assess and address the issue (Bograd, 2007a; Pressman, 1989). Efforts to ameliorate these issues and a push to address psychological and physical aggression in therapy have significantly changed the landscape of treatment in recent years (Stith, McCollum, Amanor-Boadu, & Smith, 2012) and a substantial amount of work has been done to address issues of assessment and safety in the effective treatment of couples who report psychological and physical aggression (Cleary Bradley & Gottman, 2012; Hrapczynski, Epstein, Werlinich, & LaTaillade, 2012; Todahl, Linville, Chou, & Maher-Cosenza, 2008; Todahl & Walters, 2011). Discussions about the feasibility of conjoint treatment among couples who have experienced physical or psychological aggression are ongoing, and helpful guidelines have been proposed (Stith & McCollum, 2011). Although much progress has been made regarding these issues, it is important to acknowledge that there has been controversy surrounding the question of the feasibility and ethics of whether or not clinicians should deliver conjoint therapy with clients who may become or have been physically aggressive in the past (Avis, 1992; Bograd, 2007b; Erickson, 1992; Gurman & Jacobson, 2002; Kaufman, 1992; Meth, 1992; Stith & McCollum, 2011). It is important to realize, however, irrespective of one’s stance on the issue of whether a therapist should treat a physically aggressive October 2014

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couple in conjoint therapy, it is always vital to assess the potential for physical aggression at the outset of therapy. Although many of the aforementioned manuscripts provide useful guidelines for or contraindications to conjoint couple therapy among psychologically or physically aggressive couples, data-driven predictive risk categories may function as a helpful supplemental tool for clinicians working with this population. The cutoffs presented herein for categorizing clients must still be replicated and normed before being applied to clinical practice, but the current work provides tentative guidelines for assessment. As stated earlier, there is no “gold standard” for sensitivity, specificity, and positive and negative predictive value for the prediction of physical aggression. However, specificity levels for categorizing high-risk future physical aggression are relatively high; specifically, cutoffs suggest that 80% of nonaggressive men and 99% of nonaggressive women are correctly categorized as low or moderate risk and will not perpetrate physical aggression in the future (See Table 2). For categorizing moderate risk future physical aggression, cutoffs suggest that 78% of nonaggressive women are correctly categorized as low risk. This suggests that a clinician who is particularly concerned about correctly identifying those who will not aggress should choose a high cutoff (at or above 1.38 for men and 2.75 for women). Alternately, sensitivity levels for categorizing high-risk future physical aggression suggest that 60% of men and 14% of women who are categorized as high risk will go on to perpetrate physical aggression in the future. However, when using the next lower cutoff for categorizing women at moderate risk, sensitivity jumps to 0.59 (i.e., 59% of aggressive women will be correctly categorized as aggressive). These numbers suggest that a clinician who is particularly concerned with catching more of the cases of aggression, and willing to compromise on specificity, should choose a lower cutoff (1.5 for women). See Table 2 for more details. Table 2 also offers information about positive and negative predictive value. For a clinician, these scores are particularly useful because they take the prevalence of aggression in this sample into account. In other words, these numbers represent the proportion of true positives and true negatives in this particular sample. 38% of men categorized as high risk are true positives in this sample, whereas 91% categorized a low risk are true negatives. In other words, if a man presenting for therapy is categorized as high risk based on our suggested cutoffs, in this sample, there was a 38% likelihood that he truly became aggressive. However, a male presenting for therapy who is categorized as low risk based on our cutoffs had a 91% likelihood of being nonaggressive. Alternately, 70% of women categorized as high risk are true positives, and 86% are true negatives. If a women presenting for therapy is categorized as high risk, there is a 70% chance that she did become aggressive. Women categorized as low risk had an 86% chance of being true negatives. However, using the lower cutoff for moderate aggression, 34% of women are above this cutoff were true positives, and 91% scoring below this threshold are true negatives. These statistics are particularly important for the clinician when considering safety and feasibility of conjoint therapy with a couple because they give clinicians an indicator of how worried they should be if an individual is categorized a high risk, and, alternatively, how reassured they should be if an individual is categorized as low risk. Best practices should include the assessment of both physical and psychological aggression, generally keeping in mind that those with higher levels of psychological aggression are more at risk to become physically aggressive. With these clients, whether or not they report physical aggression, it will be important for the therapist to prioritize the discussion of violence and safety planning for the future. Particularly as both negative predictive value and specificity rates are generally higher than sensitivity and positive predictive value rates, clients who are categorized as high risk require close attention to clinical guidelines of assessment and treatment as discussed in Stith et al. (2004) while clients who are categorized as low risk are significantly less worrisome. Limitations and Future Directions It must be acknowledged that our sample consists of young, Caucasian, heterosexual couples and therefore may not be generalized to other populations or cultures. Future research must aim to replicate the association between psychological and physical aggression in different populations. Another potential methodological limitation is that as all items designed to measure psychological 438

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and physical aggression are interspersed in the CTS2, the association between the scores on each subscale may be inflated due to a possible response set as participants answered all of the questions at once. Further, this work incorporates little attention to the complex characteristics and variations in severity of both psychological and physical aggression. More specifically, both psychological and physical aggression can vary in both type and severity. Unfortunately, the CTS2 scores used in the current investigation do not consider type of behavior or severity of aggression; rather, they represent total aggression sores. The lack of differentiation by type and severity of aggression limit the ecological validity of the findings to a general understanding of overall levels of psychological aggression that lead to physical aggression in the future, without the ability to differentiate which behaviors are more important indicators than others. Subsequent investigators might consider incorporating issues of severity and typology into future work. Finally, the current study did not assess for whether or not the participant had been physically aggressive in the past. It is well-known that the previous perpetration of physical aggression is a potent predictor of the future perpetration of physical aggression (O’Leary et al., 1989). Hence, the inclusion of this variable would have presumably increased the predictive power of the decision trees.

CONCLUSION To our knowledge, this is the first study in the field of aggression and intimate partner violence to use the methodology of unbiased, non-parametric conditional recursive partitioning to create decision trees that provide cutoffs of one predictor variable to predict a certain outcome. As was mentioned in the introduction, the field is not lacking in research on potential risk factors of intimate partner violence. However, there is a need for more focus on the use of those risk factors to predict future instances of physical aggression. The current methodology has utility with risk factors other than psychological aggression and can be employed with more than one risk factor at a time to identify the strongest association with future physical aggression. Likewise, regardless of the interpretation of the gender difference discussed earlier, the clinical takeaway from these data remains important; men are more likely to become physically aggressive at lower levels of psychological aggression. As always, however, it is important that clinicians attend to the seriousness of all reports of psychological aggression and do not discount certain instances because they seem less “severe” than those that lead to physical aggression.

REFERENCES Aldarondo, E., & Sugarman, D. B. (1996). Risk marker analysis of the cessation and persistence of wife assault. Journal of Consulting and Clinical Psychology, 64, 1010. doi:10.1037/0022-006X.64.5.1010. Avis, J. M. (1992). Where are all the family therapists? Abuse and violence within families and family therapy’s response. Journal of Marital and Family Therapy, 18, 225–232. doi:10.1111/j.1752-0606.1992.tb00935.x. Barling, J., O’Leary, K. D., Jouriles, E. N., Vivian, D., & MacEwen, K. E. (1987). Factor similarity of the Conflict Tactics Scales across samples, spouses, and sites: Issues and implications. Journal of Family Violence, 2(1), 37– 54. doi:10.1007/BF00976369. Bell, K. M., & Naugle, A. E. (2007). Effects of social desirability on students’ self-reporting of partner abuse perpetration and victimization. Violence and Victims, 22, 243–256. doi:10.1891/088667007780477348. Bograd, M. (2007a). Strengthening domestic violence theories: Intersections of race, class, sexual orientation, and gender. Journal of Marital and Family Therapy, 25, 275–289. doi:10.1111/j.1752-0606.1999.tb00248.x. Bograd, M. (2007b). Values in conflict: Challenges to family therapists’ thinking. Journal of Marital and Family Therapy, 18, 245–256. doi:10.1111/j.1752-0606.1992.tb00937.x. Bograd, M., & Mederos, F. (1999). Battering and couples therapy: Universal screening and selection of treatment modality. Journal of Marital and Family Therapy, 25(3), 291–312. Carney, M. M., & Barner, J. R. (2012). Prevalence of partner abuse: Rates of emotional abuse and control. Partner Abuse, 3, 286–335. doi:10.1891/1946-6560.3.3.286.

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JOURNAL OF MARITAL AND FAMILY THERAPY

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Cleary Bradley, R. P., & Gottman, J. M. (2012). Reducing situational violence in low-income couples by fostering healthy relationships. Journal of Marital and Family Therapy, 38, 187–198. doi:10.1111/j.1752-0606.2012.00288.x. Dietterich, T. (1995). Overfitting and undercomputing in machine learning. ACM computing surveys (CSUR), 27, 326–327. Doherty, W. J., & Simmons, D. S. (1996). Clinical practice patterns of marriage and family therapists: A national survey of therapists and their clients. Journal of Marital and Family Therapy, 22(1), 9–25. Erickson, B. M. (1992). Feminist fundamentalism: Reactions to Avis, Kaufman, and Bograd. Journal of Marital and Family Therapy, 18, 263–267. doi:10.1111/j.1752-0606.1992.tb00939.x. Ehrensaft, M. K., & Vivian, D. (1999). Is partner aggression related to appraisals of coercive control by a partner? Journal of Family Violence, 14(3), 251–266. Froerer, A. S., Lucas, B. M., & Brown, T. B. (2012). Current practices of intimate partner violence assessment among marriage and family therapy trainees at a university clinic. Journal of Couple & Relationship Therapy, 11(1), 16–32. Gurman, A. S., & Jacobson, N. S. (2002). Clinical handbook of couple therapy. New York, NY: Guilford Press. Hamby, S. L., & Sugarman, D. B. (1999). Acts of psychological aggression against a partner and their relation to physical assault and gender. Journal of Marriage and the Family, 61(4), 959–970. Hay, D. F., Nash, A., Caplan, M., Swartzentruber, J., Ishikawa, F., & Vespo, J. E. (2011). The emergence of gender differences in physical aggression in the context of conflict between young peers. British Journal of Developmental Psychology, 29(2), 158–175. Hines, D. A., & Saudino, K. J. (2003). Gender differences in psychological, physical, and sexual aggression among college students using the Revised Conflict Tactics Scales. Violence and Victims, 18(2), 197–217. Hothorn, T., Hornik, K., & Zeileis, A. (2006). Unbiased recursive partitioning: A conditional inference framework. Journal of Computational and Graphical Statistics, 15, 651–674. doi:10.1198/106186006X133933. Hrapczynski, K. M., Epstein, N. B., Werlinich, C. A., & LaTaillade, J. J. (2012). Changes in negative attributions during couple therapy for abusive behavior: Relations to changes in satisfaction and behavior. Journal of Marital and Family Therapy, 38, 117–132. doi:10.1111/j.1752-0606.2011.00264.x. Johnson, M. P. (2010). A typology of domestic violence: Intimate terrorism, violent resistance, and situational couple violence. Lebanon, NH: University Press of New England. Jose, A., & O’Leary, K. D. (2009). Prevalence of partner aggression in representative and clinic samples. In A. Jose, K. D. O’Leary & E. M. Woodin (Eds.), Psychological and physical aggression in couples: Causes and interventions (pp. 15–35). Washington, DC: American Psychological Association. doi: 10.1037/11880-001 Kaufman, G., Jr (1992). The mysterious disappearance of battered women in family therapists’ offices: Male privilege colluding with male violence. Journal of Marital and Family Therapy, 18, 233–243. doi:10.1111/j.1752-0606. 1992.tb00936.x. Kingsbury, M. K., & Coplan, R. J. (2012). Mothers’ gender-role attitudes and their responses to young children’s hypothetical display of shy and aggressive behaviors. Sex Roles, 66, 506–517. Leonard, K. E., & Senchak, M. (1996). Prospective prediction of husband marital aggression within newlywed couples. Journal of Abnormal Psychology, 105, 369. doi:10.1037/0021-843X.105.3.369. Marshall, L. L. (1992). Development of the severity of violence against women scales. Journal of Family Violence, 7 (2), 103–121. doi:10.1007/BF00978700. Meth, R. L. (1992). Marriage and family therapists working with family violence: Strained bedfellows or compatible partners?: A commentary on Avis, Kaufman, and Bograd. Journal of Marital and Family Therapy, 18, 257–261. doi:10.1111/j.1752-0606.1992.tb00938.x. Mossman, D. (1994). Assessing predictions of violence: Being accurate about accuracy. Journal of consulting and clinical psychology, 62, 783. Murphy, C. M., & Hoover, S. A. (1999). Measuring emotional abuse in dating relationships as a multifactorial construct. Violence and Victims, 14(1), 39–53. Murphy, C. M., & O’Leary, K. D. (1989). Psychological aggression predicts physical aggression in early marriage. Journal of Consulting and Clinical Psychology, 57, 579. doi:10.1037/0022-006X.57.5.579. Newton, R. R., Connelly, C. D., & Landsverk, J. A. (2001). An examination of measurement characteristics and factorial validity of the Revised Conflict Tactics Scale. Educational and Psychological Measurement, 61, 317–335. O’Leary, K. D., Malone, J., & Tyree, A. (1994). Physical aggression in early marriage: Prerelationship and relationship effects. Journal of Consulting and Clinical Psychology, 62, 594. doi:10.1037/0022-006X.62.3.594. O’Leary, K. D., Barling, J., Arias, I., Rosenbaum, A., Malone, J., & Tyree, A. (1989). Prevalence and stability of physical aggression between spouses: A longitudinal analysis. Journal of Consulting and Clinical Psychology, 57(2), 263. O’Leary, K. D., Smith Slep, A. M., Avery-Leaf, S., & Cascardi, M. (2008). Gender differences in dating aggression among multiethnic high school students. Journal of Adolescent Health, 42, 473–479. doi:10.1016/j.jadohealth. 2007.09.012.

440

JOURNAL OF MARITAL AND FAMILY THERAPY

October 2014

O’Leary, K. D., Smith Slep, A. M., & O’Leary, S. G. (2007). Multivariate models of men’s and women’s partner aggression. Journal of Consulting and Clinical Psychology, 75, 752. doi:10.1037/0022-006X.75.5.752. Pressman, B. (1989). Wife-abused couples. Journal of Feminist Family Therapy, 1(1), 23–43. doi:10.1300/ J086v01n01_03. Rabin, R. F., Jennings, J. M., Campbell, J. C., & Bair-Merritt, M. H. (2009). Intimate partner violence screening tools: A systematic review. American journal of preventive medicine, 36, 439–445. Rodenburg, F. A., & Fantuzzo, J. W. (1993). The measure of wife abuse: Steps toward the development of a comprehensive assessment technique. Journal of Family Violence, 8, 203–228. doi:10.1007/BF00988769. Roehl, J., O’sullivan, C., Webster, D., & Campbell, J. (2005). Intimate partner violence risk assessment validation study, final report. United States Department of Justice. Retrieved January 19, 2012. Salis, K. L., Salwen, J., & O’Leary, K. D. (2014) The predictive utility of psychological aggression for intimate partner violence. Partner Abuse, 5(1), 83–97. Saltzman, L. E., Fanslow, J. L., McMahon, P. M., & Shelley, G. A. (2002). Intimate partner violence surveillance: uniform definitions and recommended data elements: Version 1.0. Atlanta, GA: Centers for Disease Control and Prevention, National Center for Injury Prevention and Control. Saudino, K., & Hines, D. (2007). Etiological similarities between psychological and physical aggression in intimate relationships: A behavioral genetic exploration. Journal of Family Violence, 22(3), 121–129. doi:10.1007/ s10896-006-9059-6. Schafer, J., Caetano, R., & Clark, C. L. (1998). Rates of intimate partner violence in the United States. American Journal of Public Health, 88, 1702–1704. Schumacher, J. A., Feldbau-Kohn, S., Smith Slep, A. M., & Heyman, R. E. (2001). Risk factors for male-to-female partner physical abuse. Aggression and Violent Behavior, 6, 281–352. doi:10.1016/S1359-1789(00)00027-6. Schumacher, J. A., & Leonard, K. E. (2005). Husbands’ and wives’ marital adjustment, verbal aggression, and physical aggression as longitudinal predictors of physical aggression in early marriage. Journal of Consulting and Clinical Psychology, 73(1), 28. doi:10.1037/0022-006X.73.1.28. Shih, Y. S., & Tsai, H. W. (2004). Variable selection bias in regression trees with constant fits. Computational statistics & data analysis, 45, 595–607. Simpson, L. E., & Christensen, A. (2005). Spousal agreement regarding relationship aggression on the Conflict Tactics Scale-2. Psychological Assessment, 17, 423. doi:10.1037/1040-3590.17.4.423. Stets, J. E. (1990). Verbal and physical aggression in marriage. Journal of Marriage and the Family, 52(2), 501–514. doi: 10.2307/353043 Stith, S. M., & McCollum, E. E. (2011). Conjoint treatment of couples who have experienced intimate partner violence. Aggression and Violent Behavior, 16(4), 312–318. Stith, S. M., McCollum, E. E., Amanor-Boadu, Y., & Smith, D. (2012). Systemic perspectives on Intimate Partner Violence treatment. Journal of Marital and Family Therapy, 38(1), 220–240. doi:10.1111/j.1752-0606.2011. 00245.x. Stith, S. M., Smith, D. B., Penn, C. E., Ward, D. B., & Tritt, D. (2004). Intimate partner physical abuse perpetration and victimization risk factors: A meta-analytic review. Aggression and Violent Behavior, 10(1), 65–98. doi:10. 1016/j.avb.2003.09.001. Straus, M. A. (1979). Measuring intrafamily conflict and violence: The conflict tactics (CT) scales. Journal of Marriage and the Family, 41(1), 75–88. doi: 10.2307/351733 Straus, M. A., & Gelles, R.J. (1990). How violent are American families? Estimates from the National Family Violence Resurvey and other studies. Physical Violence in American families: Risk Factors and Adaptations to Violence In, 8(145), 95–112. Straus, M. A., Hamby, S.L., Boney-McCoy, S., & Sugarman, D.B. (1996). The revised conflict tactics scales (CTS2) development and preliminary psychometric data. Journal of family issues, 17, 283–316. Strobl, C., Malley, J., & Tutz, G. (2009). An introduction to recursive partitioning: Rationale, application, and characteristics of classification and regression trees, bagging, and random forests. Psychological methods, 14, 323. doi:10.1037/a0016973. Todahl, J. L., Linville, D., Chou, L. Y., & Maher-Cosenza, P. (2008). A qualitative study of intimate partner violence universal screening by family therapy interns: Implications for practice, research, training, and supervision. Journal of Marital and Family Therapy, 34(1), 28–43. Todahl, J., & Walters, E. (2011). Universal screening for intimate partner violence: A systematic review. Journal of Marital and Family Therapy, 37, 355–369.

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