Thanks for the help!

So this is the footnote:

1. Neanderthals may have had larger brains than modern humans (Ponce de León et al. 2008) and it is an open question how much Neanderthals interbred with the ancestors of modern humans. It is possible that the marginal fitness returns on cognition have leveled off sharply enough that improvements in cognitive efficiency have shifted the total resource cost of brains downward rather than upward over very recent history. If true, this is not the same as Homo sapiens sapiens becoming stupider or even staying the same intelligence. But it does imply that either marginal fitness returns on cognition or marginal cognitive returns on brain scaling have leveled off significantly compared to earlier evolutionary history

That appears to be circular reasoning. It only implies that "marginal fitness return on cognition" has leveled off if we define fitness as a function of brain size—we have no fitness measurement otherwise.

My previous suggestion, that the most important brain developments in our genus are independent of brain size, needs an explanation with a much different anchor.