Does having a twin brother make for a bigger brain?

Abstract

Objective: Brain volume of boys is larger than that of girls by w10%. Prenatal exposure to testosterone has been suggested in the masculinization of the brain. For example, in litter-bearing mammals intrauterine position increases prenatal testosterone exposure through adjacent male fetuses, resulting in masculinization of brain morphology. Design: The influence of intrauterine presence of a male co-twin on masculinization of human brain volume was studied in 9-year old twins. Methods: Magnetic resonance imaging brain scans, current testosterone, and estradiol levels were acquired from four groups of dizygotic (DZ) twins: boys from same-sex twin-pairs (SSM), boys from opposite-sex twin-pairs (OSM), girls from opposite-sex twin-pairs (OSF), and girls from same-sex twinpairs (SSF; nZ119 individuals). Data on total brain, cerebellum, gray and white matter volumes were examined. Results: Irrespective of their own sex, children with a male co-twin as compared to children with a female co-twin had larger total brain (C2.5%) and cerebellum (C5.5%) volumes. SSM, purportedly exposed to the highest prenatal testosterone levels, were found to have the largest volumes, followed by OSM, OSF and SSF children. Birth weight partly explained the effect on brain volumes. Current testosterone and estradiol levels did not account for the volumetric brain differences. However, the effects observed in children did not replicate in adult twins. Conclusions: Our study indicates that sharing the uterus with a DZ twin brother increases total brain volume in 9-year olds. The effect may be transient and limited to a critical period in childhood.

Brain SCALE: Brain Structure and Cognition: an Adolescent Longitudinal Twin Study into the Genetic Etiology of Individual Differences

“…Peper et al (2009b) 9 tw -MRI (brain volume) cross Having a male co-twin was associated with larger TB, CB and WM (corrected for own sex) compared to female co-twin; Current T and E level could not explain the enlarged brain volumes. Peper et al (2009a) 10-14 sib -MRI (volume and VBM) -E, T levels cross…”

Sex Steroids and the Organization of the Human Brain

“…Specifically, a smaller 2D:4D ratio correlates with higher FT exposure, and through the intrauterine presence of a male fetus, opposite-sex twin girls are exposed to higher FT levels than same-sex twin girls. Using the latter indirect measure of FT, earlier reports showed that total brain volume and cerebellum volume, typically found to be larger in males, were positively correlated with higher FT exposure (Peper et al, 2009).…”

“…Specifically, a smaller 2D:4D ratio correlates with higher FT exposure, and through the intrauterine presence of a male fetus, opposite-sex twin girls are exposed to higher FT levels than same-sex twin girls. Using the latter indirect measure of FT, earlier reports showed that total brain volume and cerebellum volume, typically found to be larger in males, were positively correlated with higher FT exposure (Peper et al, 2009).A recent paper by Lombardo et al (2012) provided direct evidence for an association between FT levels and sexual differentiation of brain gray matter in humans. In this pioneering study of 28 developing boys, FT levels were determined from amniotic fluid.…”

Agents Within a Developmental Complex Adaptive System: Intrauterine Male Hormones and Dental Arch Size in Humans

“…Previous twin studies supporting the TTT hypothesis have demonstrated increased masculine traits in human females from opposite-sex twin pairs as opposed to same-sex twin pairs. Examples include increased total brain volume [14], altered craniofacial growth and dental asymmetries [15], increased symptoms of Alcohol Use Disorder (AUD) [16], lower disordered eating attitudes [17] and increased tooth crown size [4,5]. These findings not only support the TTT hypothesis but also indicate that hormonal transfer may influence sensitivity of androgen receptors later in life (i.e.…”

“…Alternatively, the Twin Testosterone Transfer (TTT) hypothesis, premised on hormonal transfer between foetuses from the same pregnancy [12][13][14], allows non-invasive and cost effective assessment of the influence of prenatal testosterone on human development. Previous twin studies supporting the TTT hypothesis have demonstrated increased masculine traits in human females from opposite-sex twin pairs as opposed to same-sex twin pairs.…”