biostudies-literaturede Wit MMJacobs FoundationNIMH NIH HHSFoundation for the National Institutes of HealthDutch Research Council (NWO)National Institute of Mental HealthZonMwe12069https://www.ebi.ac.uk/biostudies/studies/S-EPMC10242923biostudies-literatureUnknown2(1)<h4>Background</h4>Having twin and non-twin siblings might influence autistic traits both prenatally and postnatally. The twin testosterone transfer hypothesis suggests that girls with a twin brother are exposed to higher levels of prenatal testosterone than girls with a twin sister, and that increased testosterone exposure masculinizes neural development and increases autistic traits. Postnatally, siblings may provide example behaviour, which could reduce autistic traits.<h4>Methods</h4>We studied pre- and postnatal influences of twin and non-twin siblings on mother and teacher-reported autistic traits in 7714 dizygotic twins. We examined the effect of sex of the proband child and of the siblings. We fitted regression models (for boys and girls separately) with sex of co-twin and having oJCPP advancesNo effects of siblings and twin testosterone transfer on autistic traits.PMC102429231RC2 MH089995R01 MH058799R01MH58799‐03RC2 MH089995Gravitation grant 024.001.003R01 MH120219024.001.003VI.Veni.191G.030056‐32‐010531003014463‐06‐001481‐08‐011451‐04‐034Gravity program 024.001.00R01MH120219912‐10‐02084920001160‐63600‐98‐834Begeer Sde Wit MMNivard MGvan Bergen EfalseNo effects of siblings and twin testosterone transfer on autistic traits.<h4>Background</h4>Having twin and non-twin siblings might influence autistic traits both prenatally and postnatally. The twin testosterone transfer hypothesis suggests that girls with a twin brother are exposed to higher levels of prenatal testosterone than girls with a twin sister, and that increased testosterone exposure masculinizes neural development and increases autistic traits. Postnatally, siblings may provide example behaviour, which could reduce autistic traits.<h4>Methods</h4>We studied pre- and postnatal influences of twin and non-twin siblings on mother and teacher-reported autistic traits in 7714 dizygotic twins. We examined the effect of sex of the proband child and of the siblings. We fitted regression models (for boys and girls separately) with sex of co-twin and having o2022-01-01T00:00:00Z2022 Mar2025-04-19T05:26:41.157Z2025-04-19T05:26:41.157ZS-EPMC102429233743149510.1002/jcv2.12069