Increases in brain size primarily occurred within individual hominin lineages over time, rather than through sudden jumps between species.
This pattern of accelerating growth became more pronounced in more recent lineages, implying an overall acceleration of relative brain size increase through human evolution. 
Brain size is typically measured as brain volume in cubic centimeters [cm³] or milliliters [mL] via MRI, or as cranial capacity/endocranial volume in skulls) varies among modern humans.
Average adult brain volume is around 1,200–1,400 cm³, with modern humans (Homo sapiens) averaging about 1,350–1,473 cm³ depending on the sample and measurement method.
Individual variation is substantial—brain size can differ by nearly twofold even among healthy adults of the same age and sex.
Males generally have larger brains than females (by ~100–150 cm³ on average), largely due to body size differences, though this persists after adjustments in many studies.
Different environmental factors drove brain vs. body size changes.
Temperature was a major predictor of body size while net primary productivity and long-term variability in precipitation correlated more with brain size — likely through indirect effects on cognitive demands and survival pressures. 
Total brain size can vary by almost twofold among typically developing humans of the same age. 
Brain size changes dynamically through early development, peaks in adolescence, and varies significantly among adults.
Research has identified nearly 1,000 genome-wide associations with brain weight.
Genes showing distinct neurodevelopmental trajectories linked to neurogenesis and cell-cell communication. 
Larger human brains show proportionally greater expansion in frontoparietal cortical networks — areas involved in reasoning and attention, than in sensory or motor systems; this pattern appears already by early childhood. 
People born in the 1970s had 6.6% larger brain volumes and nearly 15% larger cortical surface area compared to those born in the 1930s.
Brain structures including white matter, gray matter, and the hippocampus all showed increases across these generations. 
Genetics plays a major role in determining brain size, but external influences — such as health, social, cultural, and educational factors — likely also contribute. 
Brain size does not conflate size with intelligence in any simple way.
Brain structures may reflect improved brain development and health, and could represent a greater brain reserve— potentially buffering against age-related diseases like Alzheimer’s and related dementias. 
Brain organization and connectivity matter as much as size: The relationship between brain size and cognition is complex, multifactorial, and mediated by genetics, nutrition, environment, and development.
Studies of cranial capacity (from skulls) and brain volume (from MRI) have documented patterns linked to geographic ancestry and climate:
Populations with recent evolutionary history farther from the equator (e.g., East Asian or European ancestry) tend to have larger average brain volumes/cranial capacities than those nearer the equator (e.g., sub-Saharan African ancestry).
Classic large-scale skull data found averages of ~1,415 cm³ for East Asians, ~1,362 cm³ for Europeans, and ~1,268 cm³ for Africans.
Older reviews reported similar patterns: East Asians ~1,364 cm³, Europeans ~1,347 cm³, Africans ~1,267 cm³.
These differences are often ~5–10% between groups and align with broader inclines: brain size increases with distance from the equator, potentially as an adaptation to colder climates (Bergmann’s rule extended to brain tissue for heat conservation).
Modern MRI studies
These are group averages with large overlap—individual variation within any population far exceeds between-group differences.
Much of the geographic pattern ties to climate/ecogeographic factors, colder climates linked to larger bodies and brains.
Human brains have not been static:
Over the last ~3 million years of hominin evolution, brain size increased gradually.
In recent millennia (Late Pleistocene/Holocene), many studies report a decrease in average brain size compared to early modern humans. ~10–15% smaller in some populations, possibly linked to energy efficiency, domestication-like processes, or social complexity reducing selection pressure for large brains.
Conversely, within the 20th century, brain volumes appear to be increasing: One large U.S. study found people born in the 1970s had ~6.6% larger brain volumes and ~15% greater cortical surface area than those born in the 1930s, potentially due to better nutrition, health, and environment.
Brain size correlates positively with measures of intelligence (IQ or general cognitive ability).
Recent causal evidence within-family comparisons and genetic analyses like Mendelian randomization supports that larger brain size likely contributes causally to higher cognitive performance, not just confounding factors like socioeconomic status or assortative mating.
However brain size is not a direct or strong proxy for intelligence—organization, connectivity, efficiency, and specific regions such as frontoparietal networks matter more.
Modern consensus emphasizes that intelligence differences are multifaceted (genetic, environmental, cultural), and brain size alone does not explain them reliably across populations.
Within-population variation and environmental factors (nutrition, education) dominate.
Brain size shows geographic changes tied to ancestry and climate, with modest group differences overshadowed by individual variation and secular increases in recent generations.