Meeting Abstract

P1-85   -   Does the fraternal birth order effect influence handedness in humans? Hurd, PL*; Bartlett, NT; Morin, JR; University of Alberta; University of Alberta; University of Alberta phurd@ualberta.ca http://www.psych.ualberta.ca/~phurd/

The fraternal birth order effect (FBOE) is the phenomenon whereby the probability that a man self-identifies a same-sex sexual orientation in adulthood increases by 1/3 for each biological older brother. The posited mechanism behind this effect is a sub-set of mothers who mount an immune response against the Y encoded NLGN4Y protein which mounts in strength with each male child she gestates. Female fecundity effects, whereby mothers more prone to bearing gay sons are also more fecund, would also generate data consistent with the FBOE. Other effects, such as the female fecundity effect, whereby mothers more prone to bearing gay sons are also more fecund may also result in data consistent with the FBOE under some analyses. We feel that one of the more bizarre aspects of the FBOE is that no trait other than sexual orientation seems to be influenced by the FBOE. The FBOE does however interact with handedness. Several studies have demonstrated that the FBOE is limited to right handed men, left handed men do not show an FBOE. Here we examine the question of whether handedness is influenced by either, or both of the FBOE or female fecundity. We identify a further potential confound to FBOE studies, that of parental age at birth, and evidence that all three of these effects co-influence handedness in men.