A new analysis by Robin Dunbar at the University of Oxford has revived and expanded the "brain lag" hypothesis, showing that in some primate lineages—including our own—body size increased before brain size, which then dramatically overshot expected levels. The study was published in PLOS One on July 1, 2026.
The Research
Dunbar revisited the same dataset used in a 1999 analysis that originally found no statistical evidence for the brain lag hypothesis. That earlier study relied on anatomical traits and fossil dating, but since then, molecular genetic techniques have provided more accurate evolutionary timelines, and new statistical methods have emerged.
Using these advancements, Dunbar confirmed that for some primate lineages, brain size reliably lagged behind body size before catching back up to expected baseline relationships. But the story didn't end there. Instead of simply catching up, these lineages' brains then significantly overshot the expected baseline, entering what Dunbar describes as a potentially higher cognitive level.
For example, hominins and great apes experienced an exponential neural surge, with brain capacity far exceeding the expected primate baseline. This overshoot created the physical hardware required for complex reasoning. Dunbar suggests this shift from brawn to brain was a survival strategy: as ancestors moved into open, high-risk environments, they formed large, cooperative social groups to defend against predators. Managing these social networks introduced immense cognitive strain—tracking alliances, hierarchies, and bonds—which selected for larger brains, particularly an inflated neocortex. A dietary revolution, shifting from low-calorie leaves to energy-dense fruits, seeds, and nuts, provided the metabolic fuel for this neural explosion.
Why It Matters
This research highlights how social complexity and diet drove the evolution of our large brains. Understanding these evolutionary pressures can illuminate why our brains work the way they do today—why we crave social connection and why managing relationships can feel cognitively demanding. It also underscores that our cognitive abilities are not fixed; they were shaped by environmental and social challenges, and they can continue to be influenced by our experiences and choices.
What You Can Do
To support your own cognitive health, consider these evidence-based habits:
- Stay socially engaged: Maintain diverse social connections and engage in group activities that challenge your social cognition.
- Fuel your brain: Include energy-dense, nutrient-rich foods like fruits, seeds, and nuts in your diet to support brain function.
- Challenge your mind: Regularly learn new skills, solve puzzles, or play strategy games to stimulate neural growth.
Source: Neuroscience News
Curious about your own brain? Take our free adaptive IQ test or try 306 brain training levels.