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
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.
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.
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).
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 X–Y axes and would therefore frequently overlie one another. The regression lines might be useful, however, perhaps as diagnostics.
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.
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.
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.
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.
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).
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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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 | ||
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 |
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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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DOI: https://doi.org/10.1007/s10508-017-1007-4