Evaluating the twin testosterone transfer hypothesis: A review of the empirical evidence

► Hormones can transfer between fetuses in animals affecting phenotypic development. ► The existence of a similar mechanism in humans remains uncertain. ► A review finds little evidence for masculinization of behavior due to testosterone transfer. ► Some evidence consistent with transfer for perception, cognition and physiology. ► Evidence of masculinization is typically restricted to females with a male co-twin.