title: Evaluating the twin testosterone transfer hypothesis: a review of the empirical evidence.
abstract: In this paper we review the evidence that fetuses gestated with a male co-twin are masculinized in development, perhaps due to the influence of prenatal androgens: the so-called twin testosterone transfer (TTT) hypothesis. Evidence from studies of behavioral, perceptual, cognitive, morphological and physiological traits in same- and opposite-sex human twins is considered. Apart from two studies reporting increases in aspects of sensation-seeking for females with a male rather than a female co-twin, there is sparse evidence supporting the TTT hypothesis in behavioral studies. Outcomes from studies of perception (in particular otoacoustic emissions) and cognition (in particular vocabulary acquisition and visuo-spatial ability) provide more consistent evidence in support of masculinized performance in twins with a male co-twin compared to twins with a female co-twin. The outcomes favorable to the TTT hypothesis for otoacoustic emissions and visuo-spatial ability are restricted to females. Studies of physiology and morphology (e.g., brain volume, tooth size and 2D:4D ratio) also show some influence of co-twin sex, but again these effects are often restricted to female twins. Because females produce little endogenous testosterone, the effects of gestation with a male co-twin may be more pronounced in females than males. Thus, while uneven, the evidence for the TTT hypothesis is sufficient to warrant further investigation, ideally using large samples of same- and opposite-sex twins, along with control groups of same- and opposite-sex siblings when the characteristics assessed are potentially open to social influences.
authors: Neurocognitive Development Unit, School of Psychology, University of Western Australia, Australia.
authors: Aimee L
authors: AL
authors: Tapp
authors: Murray T
authors: MT
authors: Maybery
authors: Andrew J O
authors: AJ
authors: Whitehouse
mesh_terms: D001519:Behavior / Q000187:drug effects; D003071:Cognition / Q000187:drug effects; D005260:Female; D006801:Humans; D008297:Male; D010465:Perception / Q000187:drug effects; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000502:physiology; D011297:Prenatal Exposure Delayed Effects; D012737:Sex Factors; D013739:Testosterone / Q000502:physiology*; D014427:Twins / Q000523:psychology*
chemical_list: D013739:Testosterone