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Symmetric Bihemispheric Postmortem Brain Cutting to Study Healthy and Pathological Brain Conditions in Humans

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Does having a twin brother make for a bigger brain?

Jiska S Peper¹, Rachel M Brouwer, G Caroline M van Baal

¹Department of Psychiatry, Rudolf Magnus Institute of Neuroscience, University Medical Center, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands. j.s.peper@uu.nl

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Summary

Children sharing the uterus with a male twin had larger brain volumes. This effect, potentially linked to prenatal testosterone exposure, appears transient and specific to childhood.

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Area of Science:

Neuroscience
Developmental Biology
Human Genetics

Background:

Boys typically exhibit larger brain volumes than girls, with prenatal testosterone exposure implicated in brain masculinization.
In mammals, intrauterine position influences prenatal testosterone, affecting brain morphology.

Purpose of the Study:

To investigate the influence of a male co-twin on human brain volume masculinization in 9-year-old twins.
To examine the relationship between intrauterine position and brain morphology in dizygotic twins.

Main Methods:

Magnetic resonance imaging (MRI) scans were used to assess brain volumes (total brain, cerebellum, gray and white matter) in 119 dizygotic (DZ) twins.
Participants were categorized into four groups based on twin-pair sex: same-sex male (SSM), opposite-sex male (OSM), opposite-sex female (OSF), and same-sex female (SSF).
Current hormone levels (testosterone, estradiol) and birth weight were analyzed for their correlation with brain volumes.

Main Results:

Children with a male co-twin (regardless of their own sex) had significantly larger total brain (+2.5%) and cerebellum (+5.5%) volumes compared to those with a female co-twin.
Brain volumes were largest in SSM, followed by OSM, OSF, and SSF groups, with birth weight partially explaining these differences.
Current hormone levels did not account for the observed brain volume variations, and the effects were not replicated in adult twins.

Conclusions:

Sharing the uterus with a dizygotic twin brother is associated with increased total brain volume in 9-year-old children.
This masculinizing effect on brain volume appears to be transient, possibly limited to a critical developmental period in childhood.