Beyond sex differences and a male-female continuum: Mosaic brains in a multidimensional space

Area of Science:

• Neuroscience
• Developmental Biology
• Sex Differences

Background:

• The traditional view of irreversible brain masculinization in males is outdated.
• Emerging evidence suggests sex influences brains in both males and females through multiple, adaptable mechanisms.
• This complexity led to the "mosaic" hypothesis, proposing inconsistent sex-typical features within a single brain.

Purpose of the Study:

• To review animal studies supporting the mosaic hypothesis.
• To present human studies investigating brain feature consistency.
• To explore the implications of the mosaic hypothesis for understanding brain sex differences.

Main Methods:

• Magnetic resonance imaging (MRI) assessed regional volume and cortical thickness in humans.
• Postmortem analysis examined neuronal numbers in the hypothalamus.
• MRI evaluated brain changes after extreme stress exposure.

Main Results:

• Human brains exhibit unique mosaics of female-typical and male-typical features.
• Findings indicate internal inconsistency in regional brain characteristics.
• Recent data show brain features typical of one sex are also found in the other.

Conclusions:

• Human brains are best described as unique mosaics, not falling on a simple male-female continuum.
• The mosaic hypothesis provides a more accurate framework for understanding brain organization.
• Future research should consider multidimensional brain characteristics rather than binary classifications.