Children who shared the womb with a twin brother appear to have a larger brain volume at the age of nine years than children who shared with a sister. This appeared in a paper in the European Journal of Endocrinology written by neuropsychologist Jiska Peper and colleagues from University Medical Center Utrecht (UMC), and VU University Amsterdam. The article has been online since last week.
The study, led by Peper, investigated the brains of 119 children of around 9 years of age who were one of a pair of dizygotic or non-identical twins. The study was conducted using MRI brain scans. The children were divided into four groups: boys with a co-twin brother, boys with a co-twin sister, girls with a co-twin brother and girls with a co-twin sister.
Boys with a male co-twin were shown to have the largest brain volume, which is about 2.5 percent larger than boys with a female co-twin. The largest brain volume was found in boys with a male co-twin, followed by boys with a female co-twin, girls with a male co-twin, and girls with a female co-twin.
Previous studies have shown that, on average, men have an approximately ten percent larger brain volume than women. As well as this difference between the sexes, Peper and colleagues found that sharing the womb with a brother had a more masculinizing effect on the brain than sharing with a sister.
The researchers explain that their results are due to increased exposure to the male hormone testosterone before birth. Boys with a co-twin brother are exposed to the highest levels of testosterone in the womb. They have the largest brain volumes i.e. most masculinized, followed by boys with a co-twin sister. They are followed by girls with a co-twin brother and finally girls with a co-twin sister who had the smallest brain volumes.
The larger brains in children with a male co-twin cannot be explained by differences in intelligence, height, weight or hormone levels at the age of 9 years. All of these factors were comparable in all children.
Incidentally, the difference in brain volumes found in this study has not been found in adults. Peper did not find any effect in the brains of another group of twins around the age of thirty years. It is likely that exposure to hormones during puberty causes this difference in volume to disappear.