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Vilsmeier JK, Kossmeier M, Voracek M, Tran US. The fraternal birth-order effect as a statistical artefact: convergent evidence from probability calculus, simulated data, and multiverse meta-analysis. PeerJ 2023; 11:e15623. [PMID: 37609443 PMCID: PMC10441532 DOI: 10.7717/peerj.15623] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2021] [Accepted: 06/01/2023] [Indexed: 08/24/2023] Open
Abstract
The fraternal-birth order effect (FBOE) is a research claim which states that each older brother increases the odds of homosexual orientation in men via an immunoreactivity process known as the maternal immune hypothesis. Importantly, older sisters supposedly either do not affect these odds, or affect them to a lesser extent. Consequently, the fraternal birth-order effect predicts that the association between the number of older brothers and homosexual orientation in men is greater in magnitude than any association between the number of older sisters and homosexual orientation. This difference in magnitude represents the main theoretical estimand of the FBOE. In addition, no comparable effects should be observable among homosexual vs heterosexual women. Here, we triangulate the empirical foundations of the FBOE from three distinct, informative perspectives, complementing each other: first, drawing on basic probability calculus, we deduce mathematically that the body of statistical evidence used to make inferences about the main theoretical estimand of the FBOE rests on incorrect statistical reasoning. In particular, we show that throughout the literature researchers ascribe to the false assumptions that effects of family size should be adjusted for and that this could be achieved through the use of ratio variables. Second, using a data-simulation approach, we demonstrate that by using currently recommended statistical practices, researchers are bound to frequently draw incorrect conclusions. And third, we re-examine the empirical evidence of the fraternal birth-order effect in men and women by using a novel specification-curve and multiverse approach to meta-analysis (64 male and 17 female samples, N = 2,778,998). When analyzed correctly, the specific association between the number of older brothers and homosexual orientation is small, heterogenous in magnitude, and apparently not specific to men. In addition, existing research evidence seems to be exaggerated by small-study effects.
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Affiliation(s)
- Johannes K. Vilsmeier
- Department of Cognition, Emotion, and Methods in Psychology, Faculty of Psychology, University of Vienna, Vienna, Austria
| | - Michael Kossmeier
- Department of Cognition, Emotion, and Methods in Psychology, Faculty of Psychology, University of Vienna, Vienna, Austria
| | - Martin Voracek
- Department of Cognition, Emotion, and Methods in Psychology, Faculty of Psychology, University of Vienna, Vienna, Austria
| | - Ulrich S. Tran
- Department of Cognition, Emotion, and Methods in Psychology, Faculty of Psychology, University of Vienna, Vienna, Austria
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Blanchard R, Skorska MN. New Data on Birth Order in Homosexual Men and Women and a Reply to Vilsmeier et al. (2021a, 2021b). ARCHIVES OF SEXUAL BEHAVIOR 2022; 51:3319-3349. [PMID: 35713755 DOI: 10.1007/s10508-022-02362-z] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/18/2022] [Revised: 05/28/2022] [Accepted: 06/06/2022] [Indexed: 06/15/2023]
Abstract
The fraternal birth order effect (FBOE) is the repeated finding that older brothers increase the odds of homosexuality in later-born males. It has been our working assumption, based on the majority of previous studies, that a similar FBOE does not occur in females. In an elaborate quantitative review posted last year to a preprint server, Vilsmeier et al. (2021a) concluded that there is no valid evidence for an FBOE in men or women. Ablaza et al. (2022) subsequently published a study of population-level data from the Netherlands with conclusions completely opposite to those of Vilsmeier et al., namely, that there is robust evidence of an FBOE in both men and women. The present research was initially undertaken to refute the assertion of Vilsmeier et al. that there is no proof of an FBOE in men and to investigate how they obtained such a discrepant conclusion. We found evidence that the discrepancy may relate to Vilsmeier et al.'s use of the large and demonstrably unreliable sample published by Frisch and Hviid (2006). After the publication by Ablaza et al., we expanded our article to address their finding of an FBOE in women. We argue that our preferred explanation of the FBOE in men-that it reflects the progressive immunization of some mothers to Y-linked antigen by each succeeding male fetus and the concomitantly increasing effects of anti-male antibody on sexual differentiation in the brain in each succeeding male fetus-could plausibly be extended to female homosexuality.
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Affiliation(s)
- Ray Blanchard
- Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada.
| | - Malvina N Skorska
- Child and Youth Psychiatry, Centre for Addiction and Mental Health, Toronto, ON, Canada
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Blanchard R. Older Brothers and Older Sisters Odds Ratios in 36 Samples of Homosexual Males. ARCHIVES OF SEXUAL BEHAVIOR 2018; 47:829-832. [PMID: 29396612 DOI: 10.1007/s10508-018-1160-4] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2018] [Accepted: 01/24/2018] [Indexed: 06/07/2023]
Affiliation(s)
- Ray Blanchard
- Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada.
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Ippoliti R. Human Mating Theory, Migration, and Marital Dissolution. ARCHIVES OF SEXUAL BEHAVIOR 2018; 47:747-755. [PMID: 29058100 DOI: 10.1007/s10508-017-1087-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/22/2016] [Revised: 04/03/2017] [Accepted: 09/19/2017] [Indexed: 06/07/2023]
Abstract
Marriage dissolution has been extensively analyzed in the last few decades, looking for the determinants of this well-known phenomenon, but so far the relationship between female migration and marriage dissolution does not seem to have been thoroughly investigated. This work aimed to analyze the correlation between marriage dissolution and migration, which might bias the sex ratio within a given society, introducing new social and cultural values. Considering the Regions of Italy between 2002 and 2007 (i.e., 120 observations), several multiple regression models (OLS) were presented to test the proposed hypothesis, applying the robust option. The dependent variable of our empirical investigation was a marriage dissolution index, while the key explanatory variables were a sex ratio and several female rates. Based on the results, the hypothesis of a statistically significant positive relation between the influx of female immigrants and household dissolution could be supported (i.e., p < .01). However, not all migrating women are the same and a different impact is foreseeable according to the nature of this influx. Indeed, given a 1% influx of unmarried foreign females, increasing the sex ratio within society, marital dissolutions grow by 5381 for every 100,000 marriages. In the case of a 1% influx of Italian unmarried females, marital dissolutions grew by 3173 for every 100,000 marriages. Therefore, females could have specific sexual, social and cultural characteristics, which might be able to increase (or decrease) the probability of men being driven to sexual infidelity, thus leading to a higher frequency of marriage dissolutions.
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Affiliation(s)
- Roberto Ippoliti
- Department of Management, University of Turin, Corso Unione Sovietica, 218 bis, 10134, Turin, Italy.
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Zietsch BP. Reasons for Caution About the Fraternal Birth Order Effect. ARCHIVES OF SEXUAL BEHAVIOR 2018; 47:47-48. [PMID: 28971279 DOI: 10.1007/s10508-017-1086-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2017] [Accepted: 09/19/2017] [Indexed: 06/07/2023]
Affiliation(s)
- Brendan P Zietsch
- Centre for Psychology and Evolution, School of Psychology, University of Queensland, Brisbane, 4072, Australia.
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Blanchard R. Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years. ARCHIVES OF SEXUAL BEHAVIOR 2018; 47:1-15. [PMID: 28608293 DOI: 10.1007/s10508-017-1007-4] [Citation(s) in RCA: 67] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2017] [Revised: 05/15/2017] [Accepted: 05/22/2017] [Indexed: 06/07/2023]
Abstract
The fraternal birth order effect is the tendency for older brothers to increase the odds of homosexuality in later-born males. This study compared the strength of the effect in subjects from small versus large families and in homosexual subjects with masculine versus feminine gender identities. Meta-analyses were conducted on 30 homosexual and 30 heterosexual groups from 26 studies, totaling 7140 homosexual and 12,837 heterosexual males. The magnitude of the fraternal birth order effect was measured with a novel variable, the Older Brothers Odds Ratio, computed as (homosexuals' older brothers ÷ homosexuals' other siblings) ÷ (heterosexuals' older brothers ÷ heterosexuals' other siblings), where other siblings = older sisters + younger brothers + younger sisters. An Older Brothers Odds Ratio of 1.00 represents no effect of sexual orientation; values over 1.00 are positive evidence for the fraternal birth order effect. Evidence for the reliability of the effect was consistent. The Older Brothers Odds Ratio was significantly >1.00 in 20 instances, >1.00 although not significantly in nine instances, and nonsignificantly <1.00 in 1 instance. The pooled Older Brothers Odds Ratio for all samples was 1.47, p < .00001. Subgroups analyses showed that the magnitude of the effect was significantly greater in the 12 feminine or transgender homosexual groups than in the other 18 homosexual groups. There was no evidence that the magnitude of the effect differs according to family size.
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Affiliation(s)
- Ray Blanchard
- Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada.
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Blanchard R. Response to Commentaries: Meta-Analysis of Probability Samples and Other New Evidence. ARCHIVES OF SEXUAL BEHAVIOR 2018; 47:49-57. [PMID: 29256150 DOI: 10.1007/s10508-017-1134-y] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/28/2017] [Accepted: 12/05/2017] [Indexed: 06/07/2023]
Affiliation(s)
- Ray Blanchard
- Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada.
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Blanchard R, VanderLaan DP. Commentary on Kishida and Rahman (2015), Including a Meta-analysis of Relevant Studies on Fraternal Birth Order and Sexual Orientation in Men. ARCHIVES OF SEXUAL BEHAVIOR 2015; 44:1503-1509. [PMID: 25940737 DOI: 10.1007/s10508-015-0555-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Affiliation(s)
- Ray Blanchard
- Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada,
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Vrangalova Z, Savin-Williams RC. Mostly heterosexual and mostly gay/lesbian: evidence for new sexual orientation identities. ARCHIVES OF SEXUAL BEHAVIOR 2012; 41:85-101. [PMID: 22327566 DOI: 10.1007/s10508-012-9921-y] [Citation(s) in RCA: 94] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
A sample of 1,784 individuals responded to an online survey advertised on the Facebook social networking website. We explored the sexual orientation continuum by focusing on three components: self-reported sexual orientation identity, sexual attraction, and sexual partners. Results supported a 5-category classification of identity (heterosexual, mostly heterosexual, bisexual, mostly gay/lesbian, gay/lesbian) in that two added identity labels (mostly heterosexual and mostly gay/lesbian) were frequently chosen by participants and/or showed unique patterns of attraction and partners, distinct from their adjacent identities (heterosexual and bisexual, and bisexual and gay/lesbian, respectively). Those who reported an exclusive label (heterosexual, gay/lesbian) were not necessarily exclusive in other components; a significant minority of heterosexuals and the majority of gays/lesbians reported some attraction and/or partners toward their nonpreferred sex. The five identity groups differed in attraction and partners in a manner consistent with a continuous, rather than a categorical, distribution of sexual orientation. Findings also supported a sexual orientation continuum as consisting of two, rather than one, distinct dimensions (same- and other-sex sexuality). Having more same-sex sexuality did not necessarily imply having less other-sex sexuality, and vice versa. More men than women were at the exclusive ends of the continuum; however, men were not bimodally distributed in that a significant minority reported nonexclusivity in their sexuality.
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Affiliation(s)
- Zhana Vrangalova
- Department of Human Development, Cornell University, Ithaca, NY 14853-4401, USA.
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Bogaert AF. Physical development and sexual orientation in men and women: an analysis of NATSAL-2000. ARCHIVES OF SEXUAL BEHAVIOR 2010; 39:110-116. [PMID: 18719989 DOI: 10.1007/s10508-008-9398-x] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/08/2008] [Revised: 05/06/2008] [Accepted: 05/17/2008] [Indexed: 05/26/2023]
Abstract
In the present study, three physical development characteristics-weight, height, and age of menarche-were examined for their relation to sexual orientation. Participants were men and women comprising the National Survey of Sexual Attitudes and Lifestyles-2000 (N > 11,000). Participants completed self-report measures of sexual orientation, height, weight, and, for women, age of menarche. Results indicated that gay/bisexual men were significantly shorter and lighter than heterosexual men. There were no significant differences between lesbians and heterosexual women in height, weight, and age of puberty. The results add to literature suggesting that, relative to heterosexual men, gay/bisexual men may have different patterns of growth and development because of early biological influences (e.g., exposure to atypical levels of androgens prenatally). However, the present results do not support a number of studies suggesting that lesbian/bisexual women are taller and heavier than heterosexual women.
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Affiliation(s)
- Anthony F Bogaert
- Department of Community Health Sciences and Psychology, Brock University, St. Catharines, ON, Canada L2S 3A1.
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Langevin R, Langevin M, Curnoe S. Family size, birth order, and parental age among male paraphilics and sex offenders. ARCHIVES OF SEXUAL BEHAVIOR 2007; 36:599-609. [PMID: 17186126 DOI: 10.1007/s10508-006-9110-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
A sample of 1823 male paraphilics, sex offenders, and non-sex offender controls were compared on family size, birth order, and parents' ages at the time of the probands' births. Sample data were also compared to population data from Statistics Canada. The men in all groups were from larger than average Canadian families and they tended to be later born. Paraphilics and sex offenders had even larger families than offender controls. Their parents tended to be older at their birth with 34.2% of mothers and 51.3% of fathers over 30 years of age, but there were no statistically significant subgroup differences. There were also significantly more multiparous teenage mothers than expected and more paraphilics' fathers who were younger than the mothers, both factors associated in the literature with increased risk of perinatal complications and abnormalities. The confounding influences of parental age, birth order, and family size were examined and indicated the need for large samples and multivariate analysis in evaluating the role of family variables associated with paraphilics and sex offenders.
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Affiliation(s)
- Ron Langevin
- Juniper Associates, 5468 Dundas Street West, Suite 402, Etobicoke, Ontario, M9B 6E3, Canada.
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