Once again, someone at Language Log pulls out the above phrase to explain some logic problem that people have a hard time solving.
I draw your attention to it because it is an important phenomena, a sign that one is in the presence of the ideology of evolutionism*: not a mere belief that Darwin and his successors have a pretty darned good theory in hand (a proposition with which I heartily concur), but the belief that the theory is a talisman, a guide to explaining all of human experience.
Because really, what does invoking "monkey brains" have to do with explaining why humans have a hard time with multiple negations? Do rhesus monkeys and howlers have a similarly hard time, while turtles and sloths handle multiple negations quite nicely? Of course not: the more significant empirical fact** here is that not one of these animals even has the notion of a negation, and that that is pretty darned good evidence that we humans possess something somewhat different than the average monkey brain, whatever that something might be.
So, given the invocation explains absolutely nothing, why do Language Log writers repeatedly use it? It is a signaling mechanism: "Look at me: I am an evolutionist too!"
* Similarly, someone can value traditions without subscribing to traditionalism, which is respect for tradition turned into an ideology.
** I emphasis the empirical fact of the matter because evolutionists like to pretend that they are hard-nosed respecters of the empirical data, when they are, in fact, nothing of the sort, at least when it comes to their ideology.