Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

A Commentary to this article is available

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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Notes

  1. 1.

    The available evidence already indicates that the FBOE cannot be detected when family size is strongly affected by the various parental strategies (so-called stopping rules) of ceasing reproduction after one child, after one male child, or after a child of each sex (Blanchard & Lippa, 2007; Xu & Zheng, 2014, 2017; Zucker, Blanchard, Kim, Pae, & Lee, 2007). In these particular situations, which are not addressed in the present paper, neither homosexual nor heterosexual males have enough older brothers to make comparisons meaningful.

  2. 2.

    The groups included pedophiles with a sexual preference for boys and male-to-female transsexuals with a sexual preference for men. These groups can accurately be labeled as homosexual (according to biological sex), but they do not have a social identity as gay.

  3. 3.

    The studies that did not report all means were Francis (2008), Iemmola and Camperio Ciani (2009), Kangassalo, Pölkki, and Rantala (2011), and Robinson and Manning (2000).

  4. 4.

    I identified only two published studies that met the indispensable criterion 3 but failed some other criterion, and that are not mentioned in the Introduction or Discussion of this article. These were Bogaert (2005b, 2010), which did not satisfy criterion 4. I can mention, for the sake of completeness, four other studies that I did not use in my meta-analysis because they did not meet criterion 3 and sometimes also other criteria: Bearman and Brückner (2002), Bogaert (1998), McConaghy et al. (2006), and Zietsch et al. (2012).

  5. 5.

    One could also graph data this way in individual studies, plotting values for individual subjects rather than for groups. The scatterplot itself would not be as readable, because individual values could occupy only points defined by integers on the XY axes and would therefore frequently overlie one another. The regression lines might be useful, however, perhaps as diagnostics.

  6. 6.

    Full information about the regression lines, for interested readers, is as follows. Homosexual groups: R 2 = .56, F(2, 27) = 17.25, p < .0001, Constant = .32, b 1 = .18, b 2 = .07. Heterosexual groups: R 2 = .81, F(2, 27) = 55.89, p < .0001, Constant = .21, b 1 = .12, b 2 = .04.

  7. 7.

    The Older Brothers Ratio can be calculated for individual subjects, but this requires a slightly modified formula. See Blanchard (2014, Footnote 1), where this variable is labeled the Modified Ratio of Older Brothers.

  8. 8.

    Homosexual groups: R 2 = .02, F(2, 27) = .20, n.s., Constant = .67, b 1 = −.16, b 2 = .03. Heterosexual groups: R 2 = .10, F(2, 27) = 1.46, n.s., Constant = .47, b 1 = −.14, b 2 = .03.

  9. 9.

    Comparable results were obtained with an ROC analysis. The area under the curve (AUC) was .95, with a 95% confidence interval of .90–1.00. The value of the AUC may be interpreted as a 95% probability that a randomly chosen homosexual group will have a higher mean Older Brothers Ratio than will a randomly chosen heterosexual group.

  10. 10.

    Blanchard and Sheridan (1992), Blanchard, Zucker, Bradley, and Hume (1995), Blanchard, Zucker, Cohen-Kettenis, Gooren, and Bailey (1996), Bozkurt et al. (2015), Gómez-Gil et al. (2011), Green (2000), Khorashad et al. (2017), Schagen et al. (2012), VanderLaan, Blanchard, Wood, and Zucker (2014), VanderLaan et al. (2016), VanderLaan and Vasey (2011), Vasey and VanderLaan (2007).

  11. 11.

    There are at least two online interactive calculators for finding confidence intervals around odds ratios: https://select-statistics.co.uk/calculators/confidence-interval-calculator-odds-ratio/ and https://www.medcalc.org/calc/odds_ratio.php.

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Correspondence to Ray Blanchard.

Appendix

Appendix

Numbers of subjects and siblings for studies in the analysis

Original authors Subjects or sources Sexual preference Number of subjects Older brothers Older sisters Younger brothers Younger sisters
Blanchard and Bogaert (1996a) All subjects Homosexual 799 556 470 402 386
Heterosexual 3807 2223 2052 2200 2192
Blanchard and Bogaert (1996b) All subjects Homosexual 302 213 182 163 177
Heterosexual 434 209 206 287 275
Blanchard and Bogaert (1998) Offenders versus adults Homosexual 156 129 111 72 89
Heterosexual 173 154 158 185 163
Blanchard and Bogaert (1998) Offenders versus pubescents Homosexual 69 79 78 49 56
Heterosexual 127 148 119 176 151
Blanchard and Bogaert (1998) Offenders versus children Homosexual 42 42 40 36 26
Heterosexual 143 154 156 147 155
Blanchard et al. (2000) All subjects Homosexual 65 70 51 53 43
Heterosexual 152 115 105 128 123
Blanchard et al. (2006) Blanchard subsample; Table 1 Homosexual 92 98 79 61 56
Heterosexual 672 558 551 511 544
Blanchard et al. (2006) Bogaert (non-biological families); Table 1 Homosexual 280 140 115 129 120
Heterosexual 222 84 91 104 93
Blanchard et al. (2006) Bogaert (“other”); Table 1 Homosexual 267 219 174 128 134
Heterosexual 148 75 67 47 53
Blanchard and Sheridan (1992) Matched groups Homosexual 193 201 158 152 112
Heterosexual 273 134 130 202 157
Blanchard and Zucker (1994) All subjects Homosexual 569 286 256 279 283
Heterosexual 281 123 100 145 160
Blanchard et al. (1995) All subjects Homosexual 156 99 67 49 39
Heterosexual 156 65 61 65 64
Blanchard, Zucker, Cohen-Kettenis, Gooren, and Bailey (1996) All subjects Homosexual 104 103 66 58 48
Heterosexual 79 51 31 57 56
Blanchard, Zucker, Siegelman, Dickey, and Klassen (1998) All subjects Homosexual 385 205 164 185 174
Heterosexual 225 73 96 94 94
Bogaert, Bezeau, Kuban, and Blanchard (1997) All subjects Homosexual 68 52 55 33 42
Heterosexual 57 32 40 50 59
Bozkurt et al. (2015) Table 1 Homosexual 60 79 68 39 22
Heterosexual 61 27 62 35 36
Currin et al. (2015a, 2015b) 2015a, Table 1; 2015b, p. 265 Homosexual 118 61 57 67 76
Heterosexual 500 285 245 300 250
Ellis and Blanchard (2001) All subjects Homosexual 175 117 85 86 86
Heterosexual 971 494 482 484 432
Gómez-Gil et al. (2011) All subjects, p. 507 Homosexual 287 290 244 178 135
Heterosexual 38 16 24 26 22
Green (2000) p. 792, weighted means from Table 3 Homosexual 106 95 84 64 61
Heterosexual 336 201 175 202 181
Khorashad et al. (2017) All subjects Homosexual 92 213 89 21 28
Heterosexual 72 79 71 78 83
King et al. (2005) Ns vary, missing data; pp. 119–121 Homosexual 301 199 178 138 135
Heterosexual 404 190 174 198 186
Kishida and Rahman (2015) Table 1 Homosexual 905 570 534 407 380
Heterosexual 999 559 529 529 500
Schagen et al. (2012) Table 4 Homosexual 94 48 16 41 21
Heterosexual 875 298 280 298 315
Schwartz et al. (2010) Missing data; pp. 101–103, Fig. 3 caption Homosexual 677 542 447 440 467
Heterosexual 873 489 445 506 454
VanderLaan et al. (2014) Table 2 Homosexual 346 145 107 121 100
Heterosexual 210 74 65 97 84
VanderLaan et al. (2016) All subjects; unpublished data Homosexual 118 109 101 77 79
Heterosexual 143 106 79 104 100
VanderLaan and Vasey (2011) Table 1 Homosexual 133 255 226 128 138
Heterosexual 208 179 212 248 264
Vasey and VanderLaan (2007) Table 1 Homosexual 83 188 173 91 81
Heterosexual 114 140 143 93 122
Zucker and Blanchard (1994) All subjects Homosexual 98 44 48 34 28
Heterosexual 84 33 29 30 27
  1. The mean numbers of siblings in published articles were converted to total numbers of siblings by multiplying each mean by the number of subjects on which it was computed and then rounding up or down to the nearest integer

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Blanchard, R. Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years. Arch Sex Behav 47, 1–15 (2018). https://doi.org/10.1007/s10508-017-1007-4

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Keywords

  • Birth order
  • Homosexuality
  • Maternal immune hypothesis
  • Meta-analysis
  • RevMan
  • Sexual orientation
  • Transgender