There is a contrast, held for years, between two ways brains might grow. One is that body size increases first and the brain simply plays catch-up, settling back onto the expected ratio once enough generations have passed. The other possibility barely got a hearing: that catching up is only the first act, and some lineages keep going, ending up with brains far bigger than their bodies would predict.
That first idea, the “brain lag” hypothesis, had a rough two decades. In 1999,1 Robert Deaner and Charles Nunn ran the definitive test using a dataset of paired primate species, comparing how the ratio between brain and body mass changed against how long ago each pair had diverged from a common ancestor. Their logic was tidy. If body size changes first and brain size trails behind, then recently diverged species pairs should show brains that are too small for their bodies, while anciently diverged pairs should have had enough evolutionary time to correct the imbalance. Plot the residuals against divergence date and you should see a rising line.
They found nothing. No correlation for males, none for females, none when controlling for circadian patterns. The conclusion seemed airtight: brain and body size evolve together, in lockstep, under the same pressures. The idea of a lag quietly left the literature.
