Did Social Change, Not Violence, Cause Decline in Male Population?

A recent study suggests that the decline in the male population worldwide thousands of years ago may have been more due to social change rather than an unprecedented wave of violence. The French research team from CNRS, MNHN, and Paris Cité University has theorized that this decline occurred as societies transitioned from diverse reproductive systems to ones based on patrilineal lines, where men are aligned based on their fathers’ clans. Some clans had lower reproductive capacity, which impacted their entire lineage.

The decline in Y chromosome diversity was only recently identified through analysis of current men’s Y chromosomes. A study in 2015 revealed a collapse of male genetic diversity about 5,000 years ago, with one man for every 17 women involved in reproduction in Europe.

The decline was particularly severe in Europe but also affected regions like the Near East, Siberia, and Africa over a broader timeframe. Contradicting previous theories of violent clan warfare leading to the loss of Y chromosome diversity, the new study suggests that social organization was the cause.

This shift from a hunter-gatherer lifestyle to agro-pastoralism favored patrilineal systems, where clans with more power or resources were more successful in reproduction. The team collected over a thousand genomes from patrilineal populations and demonstrated significant loss of Y chromosome diversity in segmental patrilineal systems.

They attribute this social transition to the rise of agro-pastoral economies, which favored patrilocality and patrilineality. Future research aims to explore these patterns across different continents to provide region-specific insights into this historical shift.