Human brain volume may have declined by about 10% since the start of the Holocene, even as measured IQs rose across the 20th century. Experts say brain size and intelligence are only weakly correlated; possible reasons for smaller brains include the shift to agriculture, climate warming, metabolic costs and greater social specialization. Improved nutrition, education and cultural networks likely helped boost IQ scores without requiring larger brains.
If IQs Rose While Brains Shrank: Why Human Brains Got Smaller — And What It Means
An 11,000-year-old skull of a woman found in Slovakia. | Credit: Adam Ján Figeľ via Alamy
Over the last two decades some researchers have reported a modest decline in average human brain volume since the end of the last Ice Age, even as measured IQ scores rose across the 20th century. Experts say these trends are not necessarily contradictory and offer several plausible explanations for why brain size and measured cognitive performance can move in different directions.
Brain Size Is Only Weakly Linked To Intelligence
Brain volume and intelligence are only loosely related. "A larger brain does not automatically mean greater intelligence," Jeremy DeSilva, an anthropology professor at Dartmouth College, told Live Science. For example, Albert Einstein's brain was relatively small by volume, although some studies suggest unusual folding patterns in specific regions may have supported his exceptional abilities. Overall, most researchers agree there is little or no simple, direct relationship between crude measures of brain size and broad measures of intelligence in modern humans.
Do Human Brains Really Shrink?
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Not all scientists agree, but several large-sample studies point to a decline in average brain volume during the Holocene (the epoch that began about 11,700 years ago). Maciej Henneberg, emeritus professor of anthropological and comparative anatomy at the University of Adelaide, estimated a roughly 10% reduction in brain volume (about 150 ml) across the Holocene after measuring skulls from many regions. Jeremy DeSilva's lab reported a similar global trend after analyzing more than 5,000 skulls from Europe, Asia, Africa and Australia. Jeff Stibel, who contributed to those efforts and compiled additional data, also described a >10% reduction coinciding with postglacial warming.
"My research indicates that human brain size declined during the [entire span of the] Holocene by about 10% of its volume," — Maciej Henneberg.
However, other researchers find no clear, global trend. Brian Villmoare of UNLV and colleagues report that brain size has not changed meaningfully since humans attained their modern anatomy, and John Hawks cautions that many datasets overrepresent male skulls of European ancestry, making a universal conclusion difficult. Hawks also notes a partial rebound in brain size and body size in the last 150 years in industrializing nations, likely linked to improved nutrition.
Why Might Brains Shrink?
Scientists propose several, not mutually exclusive, explanations:
Some eusocial insects like ants and wasps have specific roles for each individual. It's possible that collective intelligence seen in these insects is similarly happening in humans, which may be associated with a smaller brain size. | Credit: Tomekbudujedomek via Getty Images
- Shift To Agriculture And Smaller Bodies: Farming and food production enabled larger, more sedentary communities and reduced the need for extreme physical strength. Smaller bodies require less energy, and brain size often correlates with overall body size.
- Climate Effects: Postglacial warming could favor leaner bodies and organs in warmer climates (consistent with Bergmann's and Allen's rules), indirectly affecting brain volume.
- Social Specialization And Distributed Cognition: As populations grew, tasks became more specialized and knowledge became shared across networks. DeSilva and others suggest that collective intelligence and role specialization can reduce selective pressure for larger individual brains, a pattern seen in some social insects.
- Metabolic Costs: Big brains are energetically expensive—roughly 20% of resting energy—so in times of scarce food, smaller brains may have been favored by natural selection.
Why IQ Scores Rose While Brain Size Fell
Measured IQ gains over the 20th century (the "Flynn effect") likely reflect multiple factors: improved nutrition, better health care, longer and broader education, smaller family sizes, and greater familiarity with test formats. These environmental and cultural changes can boost measured cognitive performance without requiring larger brains. In short, rising IQ scores and smaller average brain volumes are compatible if cognitive performance increasingly depends on improved developmental conditions and distributed cultural knowledge rather than sheer raw brain volume.
What This Means — And What It Doesn’t
A modest reduction in average brain volume does not mean humans are less capable. Instead, it suggests a shift in how cognition is organized: some raw individual processing capacity may have been traded for stronger cultural, technological and social networks that offload cognitive work. Whether this trade is beneficial depends on how one defines intelligence and on the long-term stability of the cultural and technological systems we rely on.
Bottom Line: The relationship between brain size and intelligence is complex. Current evidence suggests a small Holocene decline in average brain volume in some regions and samples, but that change appears compatible with rising measured IQs driven by environmental, social and cultural factors.
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