Sibling Sex Research Articles

Sibling Sex Ratio and Birth Order in Early-Onset Gender Dysphoric Adolescents

Several sibship-related variables have been studied extensively in sexual orientation research, especially in men. Sibling sex ratio refers to the ratio of brothers to sisters in the aggregate sibships of a group of probands. Birth order refers to the probands’ position (e.g., first-born, middle-born, last-born) within their sibships. Fraternal birth order refers to their position among male siblings only. Such research was extended in this study to a large group of early-onset gender dysphoric adolescents. The probands comprised 94 male-to-female and 95 female-to-male gender dysphoric adolescents. The overwhelming majority of these were homosexual or probably prehomosexual. The control group consisted of 875 boys and 914 girls from the TRAILS study. The sibling sex ratio of the gender dysphoric boys was very high (241 brothers per 100 sisters) compared with the expected ratio (106:100). The excess of brothers was more extreme among the probands’ older siblings (300:100) than among their younger siblings (195:100). Between-groups comparisons showed that the gender dysphoric boys had significantly more older brothers, and significantly fewer older sisters and younger sisters, than did the control boys. In contrast, the only notable finding for the female groups was that the gender dysphoric girls had significantly fewer total siblings than did the control girls. The results for the male probands were consistent with prior speculations that a high fraternal birth order (i.e., an excess of older brothers) is found in all homosexual male groups, but an elevated sibling sex ratio (usually caused by an additional, smaller excess of younger brothers) is characteristic of gender dysphoric homosexual males. The mechanisms underlying these phenomena remain unknown.

Evidence of intralocus sexual conflict: physically and hormonally masculine individuals have more attractive brothers relative to sisters

Intralocus sexual conflict (IASC) occurs when sex-specific selection favors genes that increase fitness in one sex and decrease fitness in the other sex. The current study was designed to explore whether IASC occurs in humans. In a sample of siblings, we identified and measured sexually dimorphic traits and hormones within each sex that are related to fitness and are likely coded for by antagonistic genes: waist-to-hip ratio (WHR) and breast size in women, WHR and bicep size (an index of muscularity) in men, and estradiol (E) and testosterone (T) in both sexes. If these traits and hormones are coded for by genes under IASC, masculine or feminine expression of traits and hormones should differentially predict brothers' and sisters' fitness. Consistent with an IASC model, both men and women who were physically masculine for their sex reported higher mate value brothers relative to sisters. Similarly, in normal-weight individuals, E levels positively predicted the mate value of sisters relative to brothers and T levels positively predicted the mate value of brothers relative to sisters. We found no evidence that individuals with indicators of high genetic quality (i.e., physically masculine men and physically feminine women) share high mate value with all siblings, regardless of sibling sex. Results are novel and demonstrate for the first time that intralocus conflict in humans may influence the fitness of related individuals.

Family history of cancer and the risk of laryngeal cancer: A case‐control study from Italy and Switzerland

Only limited data is available on the relationship between family history of laryngeal and other neoplasms and laryngeal cancer risk. We investigated the issue using data from a multicentre case-control study conducted in Italy and Switzerland between 1992 and 2009 including 852 cases with histologically confirmed laryngeal cancer and 1970 controls admitted to hospital for acute, non neoplastic conditions. Unconditional logistic regression models adjusted for age, sex, study center, education, tobacco smoking, alcohol drinking and number of siblings were used to estimate the odds ratios (ORs) of laryngeal cancer. The multivariate OR was 2.8 (95% confidence interval [CI], 1.5-5.3) in subjects reporting a first-degree relative with laryngeal cancer, as compared to subjects with no family history. The OR was higher when the relative was diagnosed before 60 years of age (OR = 3.5, 95% CI 1.4-8.8). As compared to subjects without family history, non-smokers, and moderate drinkers, the OR was 37.1 (95% CI 9.9-139.4) for current smokers, heavy drinkers, with family history of laryngeal cancer. Family history of colorectal (OR = 1.5, 95% CI 1.0-2.3) and kidney (OR = 3.8, 95% CI 1.2-12.1) cancer were also associated to an increased risk of laryngeal cancer, while no significant increase in risk was found for family history of cancer at all sites, excluding the larynx (OR = 1.1).

Persistent Staphylococcus aureus Colonization Is Not a Strongly Heritable Trait in Amish Families

About 20% of adults are persistently colonized with S. aureus in the anterior nares. Host genetic factors could contribute susceptibility to this phenotype. The objective of this study was to determine whether the phenotype of persistent S. aureus colonization aggregates in family members who live in different households. Healthy adults and their eligible same sex siblings who lived in different households were recruited from the Old Order Amish of Lancaster, Pennsylvania. All participants had two cultures of the anterior nares to determine if they were persistently colonized with S. aureus. Three hundred and ninety eight participants finished the study, of whom 166 were index cases and 232 were siblings of index cases. Eighteen per cent (71/398) of all participants and 17% (29/166) of index cases were persistently colonized with S. aureus. Twenty two per cent (8/36) of siblings of persistently colonized index cases were persistently colonized with S. aureus compared to 17% (34/196) of siblings of non-persistently colonized index cases, yielding a prevalence rate ratio of 1.28 (95% CI: 0.65–2.54, p = 0.64) and sibling relative risk of 1.25 (95% CI: 0.65–2.38, p = 0.51). The heritability of persistent colonization was 0.19±0.21 (p = 0.31). Persistent S. aureus colonization does not strongly aggregate in Amish family members in different households and heritability is low, suggesting that environmental factors or acquired host factors are more important than host genetic factors in determining persistent S. aureus colonization in this community.

Exposure to Mother's Pregnancy and Lactation in Infancy is Associated with Sexual Attraction to Pregnancy and Lactation in Adulthood

Several theories, including psychodynamic theories, sexual imprinting and early conditioning have been formulated to explain sexual development. Empirical data, however, remain insufficient for a thorough evaluation of these theories. In this study, we test the hypothesis that a critical period exists for the acquisition of sexual preferences, as suggested by empirical findings in birds and mammals (sexual imprinting). An Internet questionnaire was used. We gather data from individuals with a sexual preference for pregnant and/or lactating women, under the hypothesis that pregnancy or lactation may become sexually attractive in adulthood following an exposure to pregnant or lactating women in infancy. We find that these preferences are more common in older siblings, i.e., in individuals who have been exposed to more maternal pregnancy and lactation. This result is independent of respondent and sibling sex. In addition, only maternal pregnancies and lactations experienced between 1.5 and 5 years of age are associated with the preferences. We discuss our findings in relation to theories of sexual development and to earlier reports of birth order effects on sexual behavior. We suggest that this age range may constitute a sensitive period for the acquisition of sexual preferences.

A study on sex ratio at birth in suburban slums of Mumbai

A cross sectional study was conducted in four selected suburban slums of Mumbai to determine the sex ratio at birth and to assess the various factors related to it. Information were collected on the sex of new born babies and other socio demographic characteristics of selected couples, including number of births, history of spontaneous and induced abortions and the preferred sex of siblings. Data were collected from a total of 302 families using a pre-tested interview schedule. There were 698 births of which 351(50.3%) were males and 347 females. The sex ratio at birth was 988 females for 1000 males. There were 84 abortions of which 60(71.4%) were induced and 24 (28.6%) were spontaneous. The reason stated for induced abortions was related to sex of the child in 31(51.7%) of the cases and in 26(83.9%) of these, the abortions were induced to prevent the birth of a female child. There was a preference for male children in the study families. Gender bias and its implications are discussed.

Do brothers affect their sisters’ chances to graduate? An analysis of sibling sex composition effects on graduation from a university or a Fachhochschule in Germany

In a recent paper on gender inequality in higher education Buchman and DiPrete (2006) assume that the decrease in the gender gap in college completion in the US can partly be explained by changes in the allocation of familial resources in favour of women. However, they do not test this hypothesis empirically. In this paper I examine the effects of sibling sex composition on the graduation of women in more detail by analysing data from the German Life History Study. I assume that resources are the key issue to explaining the effects of sibling configuration on educational attainment. Tertiary education is a good case for testing sex composition effects due to the unequal distribution of resources between and within families, because the direct costs and opportunity costs of higher education are relatively high compared to those of earlier educational decisions. Accordingly, I expect that working class daughters are most likely to be disadvantaged if they are raised with brothers. The empirical results show that in fact, not the presence of a brother as such hinders educational attainment of sisters, but older brothers have a negative influence on their sisters chances of graduation. In accordance to the hypothesis, this effect is stronger for university graduation than for graduation at Fachhochschule. For social class differences in sibling effects it turns out that working class daughters are particularly less likely to graduate compared to service class daughters if there are older brothers in the family.

Mental rotation in female fraternal twins: Evidence for intra-uterine hormone transfer?

Men outperform women in mental rotation by about one standard deviation. Prenatal exposure to testosterone has been suggested as one cause. In animals it has been shown that a female fetus located between two male ones is exposed to higher levels of testosterone. It is still unclear whether intra-uterine hormone transfer exists in humans. Therefore, the influence of an intra-uterine presence of a male co twin was studied in female fraternal twins ( N = 200). Women with a male co-twin outperformed women with a female co-twin by about a third standard deviation. In a no-twin control group ( N = 200), performance of women with a slightly older sibling did not depend upon the sibling's sex. These findings provide preliminary support for the theory of an influence of prenatal testosterone on mental rotation performance.

Siblings and human capital: A comparison between Italy and France

Abstract This paper investigates how family size affects children's human capital, comparing Italy and France. We tested the in these countries, starting with the assumption that the higher the number of siblings, the fewer parental resources are available for each child, and the lower the probability that each child will successfully pursue his/her educational career. We find a negative correlation between the number of siblings and human capital. However, when the analysis is developed with a causal approach, the strength of the weakens in Italy and disappears in France. (ProQuest: ... denotes formulae omitted.) 1. Introduction Studies of the determinants of educational attainment, occupation, and income usually consider the family background as a general effect. Meanwhile, fewer researchers have investigated family size as an independent effect. In this article, we focus our attention on a set of fundamental questions regarding social inequalities that may be linked to family background characteristics, especially family size. Based on the theory, our assumption is that the higher the number of siblings, the lower the likelihood is that an individual will attain the highest levels of education (Blake 1980). Does growing up in large families affect people's educational chances? If so, is the negative on educational attainment of belonging to a large family weakened among the more advantaged social classes? Has the negative of large family size on educational attainment changed over the course of the 20th century? Are there any significant differences in the educational opportunities and life chances of first- and lastborns? When families have large numbers of children, there is often a of the resources available to each individual child. This research seeks to investigate whether, as a result of varying national family policies, the dilution effect differs between countries. In our analysis of this issue, we focus on Italy and France, two European countries where family policies and fertility choices are very different, but for which comparable data are available. The main goal of this article is to describe the statistical association between siblings and human capital, after controlling for other variables (year at birth, social class, etc.). We note, however, that some unobserved factors may also influence both family size and children's outcomes. Previous studies (see, for example, Angrist and Evans 1998) have argued that one possible source of unobserved heterogeneity is the preference among parents to have smaller families, with each child being of higher quality. The level of status anxiety for children may drive both the family size and the education of each child (Dalla Zuanna 2007). Hence, the endogeneity issue should be taken into account when evaluating the true causal of family size on education. In the final section of this article, an instrumental variable approach will be applied, using the sibling sex composition as an exogenous determinant of family size variation (Conley and Glauber 2005). This approach - which was mainly chosen because of the specificity of the data source for Italy and France - is not completely satisfactory. Consequently, this part of the research should be viewed as merely a first step towards the formulation of a more exhaustive causal approach. We intend to contribute to the literature published to date by providing the first international comparison in this area of research. It certainly is a challenge to compare two countries, especially when the surveys are not identical. Fortunately, the variables of interest examined in this paper are quite similar for the two countries, enabling us to compare different behaviors, as well as different policies. The paper is organized as follows. The next section briefly summarizes the current state of the debate about these issues in the literature, and outlines the main hypotheses. …

Otitis media incidence and risk factors in a population-based birth cohort

Otitis media is the main reason young children receive antibiotics and is the leading reason for physician visits. To characterize the incidence, recurrence and risk factors for otitis media in a population-based birth cohort. All children born in southwestern British Columbia during 1999 to 2000 were followed until the age of three years. Otitis media was defined using The International Classification of Diseases, Ninth Revision coding of physician visits, and linked with antibiotic prescription data. Information on sex, birth weight, gestational age, Aboriginal status, maternal age, older siblings, maternal smoking during pregnancy, breastfeeding initiation, neighbourhood income, female education and rural residence were obtained from vital statistics, birth hospitalizations, perinatal registry and census data. Complete risk factor information was available for 50,474 children (86% of all births). Nearly one-half of the children (48.6%) had one or more physician visits for otitis media during follow-up, and 3952 children (7.8%) met the definition for recurrent otitis media. Of the children with at least three visits during follow-up (n=7571), 73% had their initial visit during the first year of life. Aboriginal status, maternal age younger than 20 years, male sex and older siblings were the strongest risk factors identified in the adjusted conditional logistic regression models. The present study established a population-based birth cohort by linking multiple administrative databases to characterize the incidence of and risk factors for otitis media. Although the incidence of otitis media is generally low in southwestern British Columbia, important risk factors continue to be young maternal age, mothers who smoke during pregnancy and children with Aboriginal ancestry.

Educational Achievement and Socio-economic Background: Causality and Mechanisms in Senegal

This paper addresses the question of intergenerational schooling mobility in Senegal. We use an original survey conducted in 2003 that provides instruments to deal with the endogeneity of parent's education. In Senegal, school supply has been increasing a lot over the last decades, individuals who are now adults had different exposure to the schooling system, depending on where they lived when they were young and on their birth cohort. Hence, a first set of instruments describes the infrastructures available in the environment in which parents lived when they were 10 years old. Moreover, variation in education demand for a child is also driven by his/her position among his/her siblings, since older children tend to be less educated in West Africa. Being the first born is random but implies different educational outcomes than other birth ranks. Hence, the second set of instruments describes whether the parents were the first born among their same sex siblings. The estimated effect of father's education is more than double when its endogeneity is accounted for. Unexpectedly, mother's education comes out as a lesser determinant. We then focus on the understanding of the channels through which parental education affects children's schooling. We present the results pointing at the importance of the direct impact of parental education relative to the effect passing through wealth or household activity choices.

Birth Order and Ratio of Brothers to Sisters in Spanish Transsexuals

Three Western studies have shown that male-to-female (MF) homosexual transsexuals tend to be born later than their siblings and to come from sibships with more brothers than sisters. The objective of this study was to determine whether these variables would be replicated in 530 MF and female-to-male (FM) Spanish transsexuals according to sexual orientation. The results showed that MF homosexual transsexuals had significantly more older brothers than the non-homosexual MF group. Compared with the expected rates in the general population, birth order was significantly higher in both MF (Slater's Index = 0.59; Fraternal Index = 0.61; Sororal Index = 0.58) and FM homosexual transsexuals (Slater's Index = 0.65; Fraternal Index = 0.68; Sororal Index = 0.67), and sibling sex ratio was significantly higher than expected in homosexual MF (sex ratio = 0.55) but not in homosexual FM transsexuals. No significant differences were found in the non-homosexual subgroups. The replication of the later birth order and sibling sex-ratio effect in MF homosexual transsexuals corroborates previous findings in a variety of groups from different cultures and may suggest a common mechanism underlying the etiology of transsexualism.

Sibling sex ratio of individuals diagnosed with autism spectrum disorder as children

To study the sex ratio (proportion of males) in siblings of individuals diagnosed with autism spectrum disorders (ASDs) as children. In the current study, we extended previous studies dealing with the androgen theory of autism and examined sex ratios in the siblings of 326 individuals with ASD (245 males, 81 females) who had been consecutively assessed at two Danish university clinics of child psychiatry during the 25-year period from 1960 to 1985. Among the 513 siblings, 300 were males and 213 females. This yields a sex ratio of 0.585, which is significantly higher than the Danish live-birth sex ratio over the same period (0.514, p=0.001). The sibling sex ratio was not associated with the IQ in the autistic probands. Our findings suggest a potential indirect confirmation of the androgen theory of autism.

PW01-244 - A retrospective sibling study of childhood adjustment in adults with substance use disorders

AimTo retrospectively identify differences in antisocial and delinquent behavior between siblings discordant for serious substance use disorders.BackgroundResearch has suggested that people who develop serious substance use disorders have delinquent traits as children prior to using any illicit drugs.MethodA retrospective survey was conducted of 50 pairs of adult siblings who were discordant for serious substance use disorders. The self-report early delinquency scale (SRED) was used to retrospectively assess delinquent traits in childhood and adolescence.ResultsThe mean SRED 29 (illegal item) score in treatment seeking opiate dependent people (mean age= 33.5 years) was 19.6 (SE=0.9) compared to 4.8 (SE=0.6) in same sex, non-substance dependent siblings (p< 0.0001; paired Wilcoxon test). The mean SRED 58 (norm violation) score in treatment seeking opiate dependent people was 36.8 (SE=1.7) compared to 10.5 (SE=1.4) in same sex siblings (p< 0.0001; paired Wilcoxon test). 40% of patients were expelled from school compared to 12% of siblings (Yates-corrected Chi2 = 8.78; p = 0.003). 96% of patients reported problems with the police as adolescence compared to 48% of controls (Yates-corrected Chi2 = 26.24; p < 0.0001). Police problems in substance users pre-dated first use of illicit substances by 5.5 years.ConclusionThe study used a validated instrument and objective reports to confirm significantly higher rates of delinquent behavior and social dysfunction in childhood for substance users compared to non-substance dependent siblings. The delinquent behaviors usually predated serious substance misuse.

Impact of Sibship Size, Birth Order and Sex Composition on School Enrolment in Urban Turkey*

AbstractThis paper investigates, in a unified framework, the effects of sibship size, birth order and sibling sex composition on children's school enrolment in urban Turkey. We utilize an instrumental variable estimation method to address parents’ joint fertility and schooling decisions using twin births as instruments. We find no causal impact of sibship size on school enrolment. However, there is evidence for a parabolic impact of birth order where middle‐born children fare worse. Sex composition of siblings matters only for female children. Our finding that birth order and sibling sex composition matter more for poorer households suggests that scarce financial resources play an important role in bringing about the sibling composition effects.

Sibling Sex Composition and Educational Outcomes: A Review of Theory and Evidence for the UK

AbstractStudies from US data have found mixed results regarding sibling sex composition effects and educational outcomes. Some researchers have found that sisters hurt women's education, others have found the opposite, and some have found no effects. This paper reviews the theoretical foundations for sibling sex composition effects and tests the theories using British data, on a sample of children born between 3 and 9 March 1958, from the National Child Development Survey. Methods from previous studies are replicated and extensions considered for ability, credit constraints, and family size. No sibling sex composition effects are found, but estimation results support the quantity–quality model.

Trade-offs in modern parenting: a longitudinal study of sibling competition for parental care

Evolutionary and economic models of the family propose that parents face a fundamental trade-off between fertility and investment per offspring. However, tests of this hypothesis have focused primarily on offspring outcomes rather than direct measures of parental investment. Existing studies of parenting also suffer a number of methodological problems now recognized as common sources of error in sociodemographic studies. Here, we present a more definitive picture of the effects of family structure on parental care by analyzing an extensive longitudinal dataset of contemporary British families (the Avon Longitudinal Study of Parents and Children). Unlike other studies, we simultaneously track maternal and paternal behaviors within the same family and consider variation both across time and between distinct population subgroups. Parental investment was measured as frequency of engagement in key care activities over the first decade of life. For both parents, larger family size was traded off against investment per offspring, representing the strongest explanatory variable considered in our analysis. However, contrary to the predictions of traditional quantity–quality trade-off models, increasing family socioeconomic status did not alleviate this effect. In fact, for paternal care in particular, increases in wealth and education created stronger trade-offs. We also demonstrate that large sibships were particularly costly for later-born offspring. Sex of siblings did not influence parental care, however maternal investment was biased towards daughters and paternal investment biased towards sons. Unrelated father figures were also associated with lower investment from both parents. Results are discussed in relation to parental investment theory and evolutionary models of modern low fertility.

Self-representations in Early Adolescence: Variations in Sibling Similarity by Sex Composition and Sibling Relationship Qualities

Self-representations play an important role in adolescent development. This study compared self-representations for siblings and explored whether sibling relationship characteristics are associated with similarities or differences in sibling self-concepts. We examined self-representations of 438 adolescent sibling dyads (M age younger sibling = 11.6 years, M age older = 14.3 years), finding that siblings are, on average, similar in their self-representations. This similarity varied, however, depending on sex composition and sibling relationship qualities. Results indicated that sibling modeling, warmth, and conflict were especially influential in predicting sibling resemblance vs. dissimilarity. © 2009 Blackwell Publishing Ltd.

Litter environment affects behavior and brain metabolic activity of adult knockout mice.

In mammals, the formative environment for social and anxiety-related behaviors is the family unit; in the case of rodents, this is the litter and the mother-young bond. A deciding factor in this environment is the sex ratio of the litter and, in the case of mice lacking functional copies of gene(s), the ratio of the various genotypes in the litter. Both Sex and Genotype ratios of the litter affect the nature and quality of the individual's behavior later in adulthood, as well as metabolic activity in brain nuclei that underlie these behaviors. Mice were raised in litters reconstituted shortly after to birth to control for sex ratio and genotype ratio (wild type pups versus pups lacking a functional estrogen receptor α). In both males and females, the Sex and Genotype of siblings in the litter affected aggressive behaviors as well as patterns of metabolic activity in limbic nuclei in the social behavior network later in adulthood. Further, this pattern in males varied depending upon the Genotype of their brothers and sisters. Principal Components Analysis revealed two components comprised of several amygdalar and hypothalamic nuclei; the VMH showed strong correlations in both clusters, suggesting its pivotal nature in the organization of two neural networks.

Meet the Parents? The Causal Effect of Family Size on the Geographic Distance between Adult Children and Older Parents

An emerging question in demographic economics is whether there is a link between family size and the geographic distance between adult children and elderly parents. Given current population trends, understanding how different configurations of family size and sibship influence patterns of child-parent proximity is vitally important, as it impacts on issues such as intergenerational care and everyday mobility. It may be the case, for example, that larger families enable the responsibility of care for older parents to be shared among more siblings, possibly decreasing individual involvement and relaxing constraints on geographic mobility. However, there is no causal evidence to date on this issue. This study is the first attempt to estimate the causal effect of sibship size on the geographic distance between older parents and adult children by using a large administrative data set from Sweden. We find a positive association between sibship size and child-parent geographic distance. However, when we use multiple births and sibship sex composition as instruments for family size, we do not find any evidence that the observed positive relationship represents a causal effect. Given that family sizes are continuing to fall in many developed countries, our findings suggest that the trend towards smaller families will not necessarily result in adult children being more constrained in terms of their geographic location decisions, at least in countries with extensive state-provision of elderly care.