Why do women have less synapses / neurons / brain mass than men despite having the same average IQ?
In brief, so far as we can tell size matters far less than brain architecture.
Consider that humans have only about half as many cortical neurons as a number of whales, yet are clearly far more intelligent.
While we are still far from a complete understanding of the details of how intelligence actually works, neuron count is only part of the story.
For humans in particular, there is also a selection argument that would lead us to expect that relatively small differences in neuron count (e.g., between male and female average) are not particularly significant. General intelligence (not &quot;IQ&quot;) appears to be a critical aspect of human selection, and thus under significant evolutionary pressure for both sexes. If a small adjustment in neuron count made a significant different in intelligence, cranial and cortical scale is a relatively simple morphogenetic adjustment, and we would expect both sexes to scale until they hit a more similar scaling boundary. Since that has not happened, it is reasonable to conclude that neuron count is at least not a major factor in intelligence.