Journal of Medical Genetics, 1979, 16, 24-31
Inbreeding effects on fertility and sterility in southern India 1 P. S. S. RAO AND S. G. INBARAJ From the Department of Biostatistics, Christian Medical College, Vellore N.A., Tamil Nadu, India SUMMARY The effects of consanguineous marriages on couples' fertility and sterility were explored through an interview survey of 20 626 women, chosen randomly from the rural and urban areas of the North Arcot District of Tamil Nadu State. Qualified women investigators obtained relevant information about reproductive performances of all married women resident in well defined rural and urban samples chosen randomly from North Arcot District. For each marriage, a family pedigree was drawn, extending upwards to two earlier generations on both sides of each spouse, in order to determine the existence and type of consanguinity involved. Of marriages in rural areas, 46 9 % were consanguineous, and in urban areas, 29 * 1 %. In more than 80 % of the consanguineous marriages, the spouses were first cousins or more closely related. The extent of primary and secondary sterility and the level of fertility were examined in relation to each type of consanguineous marriage with the duration of the marriage and the age of the woman. The frequency of primary sterility appeared to be lower in the consanguineous marriages compared to that in the non-consanguineous marriages. However, the differences were only marginal and only occasionally attained statistical significance. No trends were seen in the degree of consanguineous relationship, and there did not appear to be any association with the duration of marriage or the age of the woman. The frequencies of secondary sterility did not differ significantly in consanguineous marriages in either the rural or the urban areas. No consistent associations were observed with degree of relationship. There were no specific associations in terms of the duration of marriage or the age of the woman observed in the frequencies of secondary sterility. The mean levels of fertility were slightly raised among the consanguineous marriages and attained significance merely because of the large sample sizes involved. These findings are discussed and compared with relevant published work. Comparisons are made difficult because of paucity of data based on community studies, and also because great differences exist in the methodology adopted by various investigators. The findings from the present study seem to show that long-term inbreeding results in only marginal or non-significant effects on fertility of inbred populations. -
because of selection against particularly lethal genes, others, such as blood group compatibility, result in higher fertility, especially among inbred groups. In exploring the relationship of inbreeding to fertility, it is necessary to consider both the extent and the duration of the practice of consanguinity, so as to distinguish between short term and long term effects (Sanghvi, 1966). In contrast to any other part of the world, consanguineous marriages are quite popular in south India, the practice dating back several centuries (Rao et al., 1972). This situation provided an opportunity to explore the long term effects of inbreeding on several facets of human
Published reports on the heritability of fertility and on variations in fertility among inbred families are few and inconclusive (Pearson et al., 1899; Imaizumi et al., 1970; Williams and Williams, 1974). Variations in fertility and sterility can be caused by several environmental factors or by specific genetic mechanisms. While some of these factors lead to greater reproductive losses and incapacity to conceive Financed by the National Center for Health Statistics, Public Health Service, Department of Health, Education and Welfare, Washington D.C. 20201, USA, under Agreement No. 01-657-2 NCHS-IND-7. Received for publication 7 June 1978
I
24
Inbreeding effects on fertility and sterility in southern India reproduction and development. An earlier paper dealt with inbreeding effects on reproductive losses and congenital malformations, on the basis of a comprehensive community study carried out in Tamil Nadu State during 1969 to 1975 (Rao and Inbaraj, 1977a). In this paper the effects of consanguineous marriages on couples' fertility and sterility are described.
Materials and methods The study was carried out in one of the 14 districts in the state of Tamil Nadu (formerly known as Madras State). This district (North Arcot) has a population of 3 76 million (Tamil Nadu District Census, 1971) in about 14 taluks. Each taluk has 4 to 5 rural administrative sub-units known as Panchayat Unions or Community Development Blocks. One such Panchayat Union (K. V. Kuppam Block) in Gudiyatham Taluk was chosen as the rural sample. A random selection of representative segments of Vellore town, the headquarters of North Arcot District, provided the urban sample. The demographic characteristics of the district are fairly typical of those in other parts of the state. The principal local language is Tamil. In both the rural and urban areas, 85 to 90 % of the population are Hindus, about 8 % are Muslims, 2 % are Christians, and roughly 1 % belong to other religious groups (mostly Jains and Buddhists). The predominant occupation is agriculture and the literacy rate is only 35 %. Women interviewers, who were fluent in the local language (Tamil), and who had undergone the auxiliary-nurse midwives' course or some other basic course in health science, were appointed and given intensive training for the research project and in interview methods. Each interviewer was responsible for about 4000 people in the rural area and about 5000 in the urban area. For every 4 interviewers, there was a supervisor (female) and a co-ordinator
(male).
The first stage of the study concentrated on mapping all houses and identifying each family. All the married women in these families were initially interviewed for general demographic details, and at subsequent visits subjected to an intensive interview, using a special questionnaire, to elicit information on:
(a) ages at puberty, marriage, and consummation of marriage;
25
drawing of family pedigrees and on interview techniques required to obtain data on consanguinity. If there was a consanguineous marriage, a pedigree was drawn extending upwards to two earlier generations on both sides of each spouse. This diagram was checked by the woman, and by at least one other adult familiar with the couple. Several pedigree diagrams depicting various degrees of closeness of relationship were drawn and given to each investigator for reference. On the basis of reinterview by supervisory personnel, it was found that misclassification of a consanguineous marriage as nonconsanguineous or vice versa was practically zero. Similarly, misclassification of uncle-niece and first cousin marriage were rare. Only for relationships beyond that of first cousin were there some minor discrepancies. In the present analyses, all consanguineous types beyond that of first cousin matings have been grouped into one class. Great emphasis was placed on obtaining information on every pregnancy and not just on those ending in a live birth. This required great patience and considerable tact as well as competence on the part of the investigator. Various kinds of memory aids were used to assist in identification of every pregnancy or to check the sequence of reported pregnancies. Significant events in the life of the woman and of members of her family, ages of her children now surviving, and their educational or occupational status were used in the conversation to finally determine the total of all pregnancies. Of all married women, 5 % were reinterviewed by supervisory personnel. Relevant data were coded, transferred to 80-column punch cards, and processed through IBM computer systems.
Definitions Any married woman who had not produced a liveborn baby after consummation of marriage and unprotected sexual activity was considered to have 'primary sterility'. Any married woman who had no issue after the first liveborn baby, despite continuance of marriage and unprotected sexual activity, was considered to have 'secondary sterility'. The total number of live births that a woman had produced was taken as her level of fertility. The mean number of all live births in a group was taken as the average fertility for that group. All these indicators were further classified in terms of the age of the woman and the duration of her married life.
(b) consanguinity of marriage; and (c) details on each of past pregnancies, their out- Results come, and the survival of the offspring. , The investigators were given thorough training in
The average inbreeding coefficients in the rural and urban areas were 0-0371 and 0-0204, respectively;
26
Rao and Inbaraj
Table 1 Number of women studied by type of consanguinity and duration of marriage: rural and urban Relationship
Total
Duration of marriage (y)
0-4
5-9
10-14
15-19
20 or more
Rural Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
1395 1106 176 555 375 2501
1076 945 122 504 319 2021
996 920 119 461 340 1916
727 761 109 401 251 1488
1975 1727 206 965 556 3702
6169 5459 732 2886 1841 11 628
Urban Non-consanguineous Consanguineous Beyond first cousins First coLtsins Uncle-niece Total
1605 594 117 348 129 2199
1277 511 89 308 114 1788
1160 447 89 258 100 1607
848 380 57 241 82 1228
1488 688 106 418 164 2176
6378 2620 458 1573 589 8998
Table 2 Number of women studied by type of consanguinity and year of birth: rural and urban Relationship
Rural Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
Urban Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
Total
Year of birth
Before 1920
1920-1929
1930-1939
1940-1949
1950 and later
1012 675 72 360 243 1687
689 670 81 381 208 1359
1254 1326 184 716 426 2580
2036 1862 250 962 650 3898
1178 926 145 467 314 2104
6169 5459 732 2886 1841 11 628
665 254 38
662 314 44 199 71 976
1625 661 111 409 141 2286
2441 955 178 559 218 3396
985 436 87 255 94 1421
6378 2620 458 1573 589 8998
151 65 919
Table 3 Percentage of women with primary sterility by consanguinity and duration of marriage: rural and urban Relationship Rural Non-consanguineous
Consanguineous Beyond first cousins First cousins Uncle-niece Total
Duration of marriage (y) 0-4
5-9
10-14
15-19
20 or more
62.1 64-1 71.4 63-5 61.5 63.0
10.6 10-2 10.3 6.8 15.3 10.4
5-0 3-5 3.5 4-1 2-8 4-3
3.9 3-7 2.9
3.7 3-1 2-0
4-2 3.3 3.8
3-4 3-0 3-4
54.3 61.3* 56.4 62.1 63.3 56.2
12.1 13.4 16-7 12.1 11.2 12.3
9.2 6-0*
5.9 5.6
6*0
7.4
4.8 3-6 2.0 4.3 2.6 4.4
Urban
Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
5.4 7.4 8.3
4.9 6-6 5.8
*Difference between non-consanguineous and consanguineous significant, P
Inbreeding effects on fertility and sterility in southern India 1 P. S. S. RAO AND S. G. INBARAJ From the Department of Biostatistics, Christian Medical College, Vellore N.A., Tamil Nadu, India SUMMARY The effects of consanguineous marriages on couples' fertility and sterility were explored through an interview survey of 20 626 women, chosen randomly from the rural and urban areas of the North Arcot District of Tamil Nadu State. Qualified women investigators obtained relevant information about reproductive performances of all married women resident in well defined rural and urban samples chosen randomly from North Arcot District. For each marriage, a family pedigree was drawn, extending upwards to two earlier generations on both sides of each spouse, in order to determine the existence and type of consanguinity involved. Of marriages in rural areas, 46 9 % were consanguineous, and in urban areas, 29 * 1 %. In more than 80 % of the consanguineous marriages, the spouses were first cousins or more closely related. The extent of primary and secondary sterility and the level of fertility were examined in relation to each type of consanguineous marriage with the duration of the marriage and the age of the woman. The frequency of primary sterility appeared to be lower in the consanguineous marriages compared to that in the non-consanguineous marriages. However, the differences were only marginal and only occasionally attained statistical significance. No trends were seen in the degree of consanguineous relationship, and there did not appear to be any association with the duration of marriage or the age of the woman. The frequencies of secondary sterility did not differ significantly in consanguineous marriages in either the rural or the urban areas. No consistent associations were observed with degree of relationship. There were no specific associations in terms of the duration of marriage or the age of the woman observed in the frequencies of secondary sterility. The mean levels of fertility were slightly raised among the consanguineous marriages and attained significance merely because of the large sample sizes involved. These findings are discussed and compared with relevant published work. Comparisons are made difficult because of paucity of data based on community studies, and also because great differences exist in the methodology adopted by various investigators. The findings from the present study seem to show that long-term inbreeding results in only marginal or non-significant effects on fertility of inbred populations. -
because of selection against particularly lethal genes, others, such as blood group compatibility, result in higher fertility, especially among inbred groups. In exploring the relationship of inbreeding to fertility, it is necessary to consider both the extent and the duration of the practice of consanguinity, so as to distinguish between short term and long term effects (Sanghvi, 1966). In contrast to any other part of the world, consanguineous marriages are quite popular in south India, the practice dating back several centuries (Rao et al., 1972). This situation provided an opportunity to explore the long term effects of inbreeding on several facets of human
Published reports on the heritability of fertility and on variations in fertility among inbred families are few and inconclusive (Pearson et al., 1899; Imaizumi et al., 1970; Williams and Williams, 1974). Variations in fertility and sterility can be caused by several environmental factors or by specific genetic mechanisms. While some of these factors lead to greater reproductive losses and incapacity to conceive Financed by the National Center for Health Statistics, Public Health Service, Department of Health, Education and Welfare, Washington D.C. 20201, USA, under Agreement No. 01-657-2 NCHS-IND-7. Received for publication 7 June 1978
I
24
Inbreeding effects on fertility and sterility in southern India reproduction and development. An earlier paper dealt with inbreeding effects on reproductive losses and congenital malformations, on the basis of a comprehensive community study carried out in Tamil Nadu State during 1969 to 1975 (Rao and Inbaraj, 1977a). In this paper the effects of consanguineous marriages on couples' fertility and sterility are described.
Materials and methods The study was carried out in one of the 14 districts in the state of Tamil Nadu (formerly known as Madras State). This district (North Arcot) has a population of 3 76 million (Tamil Nadu District Census, 1971) in about 14 taluks. Each taluk has 4 to 5 rural administrative sub-units known as Panchayat Unions or Community Development Blocks. One such Panchayat Union (K. V. Kuppam Block) in Gudiyatham Taluk was chosen as the rural sample. A random selection of representative segments of Vellore town, the headquarters of North Arcot District, provided the urban sample. The demographic characteristics of the district are fairly typical of those in other parts of the state. The principal local language is Tamil. In both the rural and urban areas, 85 to 90 % of the population are Hindus, about 8 % are Muslims, 2 % are Christians, and roughly 1 % belong to other religious groups (mostly Jains and Buddhists). The predominant occupation is agriculture and the literacy rate is only 35 %. Women interviewers, who were fluent in the local language (Tamil), and who had undergone the auxiliary-nurse midwives' course or some other basic course in health science, were appointed and given intensive training for the research project and in interview methods. Each interviewer was responsible for about 4000 people in the rural area and about 5000 in the urban area. For every 4 interviewers, there was a supervisor (female) and a co-ordinator
(male).
The first stage of the study concentrated on mapping all houses and identifying each family. All the married women in these families were initially interviewed for general demographic details, and at subsequent visits subjected to an intensive interview, using a special questionnaire, to elicit information on:
(a) ages at puberty, marriage, and consummation of marriage;
25
drawing of family pedigrees and on interview techniques required to obtain data on consanguinity. If there was a consanguineous marriage, a pedigree was drawn extending upwards to two earlier generations on both sides of each spouse. This diagram was checked by the woman, and by at least one other adult familiar with the couple. Several pedigree diagrams depicting various degrees of closeness of relationship were drawn and given to each investigator for reference. On the basis of reinterview by supervisory personnel, it was found that misclassification of a consanguineous marriage as nonconsanguineous or vice versa was practically zero. Similarly, misclassification of uncle-niece and first cousin marriage were rare. Only for relationships beyond that of first cousin were there some minor discrepancies. In the present analyses, all consanguineous types beyond that of first cousin matings have been grouped into one class. Great emphasis was placed on obtaining information on every pregnancy and not just on those ending in a live birth. This required great patience and considerable tact as well as competence on the part of the investigator. Various kinds of memory aids were used to assist in identification of every pregnancy or to check the sequence of reported pregnancies. Significant events in the life of the woman and of members of her family, ages of her children now surviving, and their educational or occupational status were used in the conversation to finally determine the total of all pregnancies. Of all married women, 5 % were reinterviewed by supervisory personnel. Relevant data were coded, transferred to 80-column punch cards, and processed through IBM computer systems.
Definitions Any married woman who had not produced a liveborn baby after consummation of marriage and unprotected sexual activity was considered to have 'primary sterility'. Any married woman who had no issue after the first liveborn baby, despite continuance of marriage and unprotected sexual activity, was considered to have 'secondary sterility'. The total number of live births that a woman had produced was taken as her level of fertility. The mean number of all live births in a group was taken as the average fertility for that group. All these indicators were further classified in terms of the age of the woman and the duration of her married life.
(b) consanguinity of marriage; and (c) details on each of past pregnancies, their out- Results come, and the survival of the offspring. , The investigators were given thorough training in
The average inbreeding coefficients in the rural and urban areas were 0-0371 and 0-0204, respectively;
26
Rao and Inbaraj
Table 1 Number of women studied by type of consanguinity and duration of marriage: rural and urban Relationship
Total
Duration of marriage (y)
0-4
5-9
10-14
15-19
20 or more
Rural Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
1395 1106 176 555 375 2501
1076 945 122 504 319 2021
996 920 119 461 340 1916
727 761 109 401 251 1488
1975 1727 206 965 556 3702
6169 5459 732 2886 1841 11 628
Urban Non-consanguineous Consanguineous Beyond first cousins First coLtsins Uncle-niece Total
1605 594 117 348 129 2199
1277 511 89 308 114 1788
1160 447 89 258 100 1607
848 380 57 241 82 1228
1488 688 106 418 164 2176
6378 2620 458 1573 589 8998
Table 2 Number of women studied by type of consanguinity and year of birth: rural and urban Relationship
Rural Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
Urban Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
Total
Year of birth
Before 1920
1920-1929
1930-1939
1940-1949
1950 and later
1012 675 72 360 243 1687
689 670 81 381 208 1359
1254 1326 184 716 426 2580
2036 1862 250 962 650 3898
1178 926 145 467 314 2104
6169 5459 732 2886 1841 11 628
665 254 38
662 314 44 199 71 976
1625 661 111 409 141 2286
2441 955 178 559 218 3396
985 436 87 255 94 1421
6378 2620 458 1573 589 8998
151 65 919
Table 3 Percentage of women with primary sterility by consanguinity and duration of marriage: rural and urban Relationship Rural Non-consanguineous
Consanguineous Beyond first cousins First cousins Uncle-niece Total
Duration of marriage (y) 0-4
5-9
10-14
15-19
20 or more
62.1 64-1 71.4 63-5 61.5 63.0
10.6 10-2 10.3 6.8 15.3 10.4
5-0 3-5 3.5 4-1 2-8 4-3
3.9 3-7 2.9
3.7 3-1 2-0
4-2 3.3 3.8
3-4 3-0 3-4
54.3 61.3* 56.4 62.1 63.3 56.2
12.1 13.4 16-7 12.1 11.2 12.3
9.2 6-0*
5.9 5.6
6*0
7.4
4.8 3-6 2.0 4.3 2.6 4.4
Urban
Non-consanguineous Consanguineous Beyond first cousins First cousins Uncle-niece Total
5.4 7.4 8.3
4.9 6-6 5.8
*Difference between non-consanguineous and consanguineous significant, P
