I have some thoughts, yeah.

So there is extremely solid evidence of cognitive differences.

This is uncomfortable to liberals, but who cares. That's nothing to do with is it true or not.

There was a guy called Charles Spearman about 100 years ago who gave groups tests on various cognitive abilities:

• General knowledge. What's the capital of Bolivia?

• Visualising: which of these shapes can be made from these shapes?

• Give people 100 arithmetic questions like 13×37 to answer in three minutes. The questions are easy, but how many can you get in three minutes? It's a test of speed.

• Vocabulary: do you know what 'bellicose' means?

It turns out there is a correlation between a person's scores on these categories. Someone who scores high will score high across the board. Someone who scores low will score low across the board. Of course the correlation is not 100%, people have comparative strengths/weaknesses.

There's no negative correlation, which would be a trade-off, e.g. being sharp verbally reduces your visual intelligence. Cross-culturally, girls and boys score the same.

This finding is extremely well replicated and solid. It is true cross-culturally: find a classroom of kids in Kerry, Caracas, Korea, and there'll be bright/sharp kids in the class, and not-so-bright ones. That's a robust finding regardless of how it fits anyone's political views.

So yeah, we can test cognitive ability pretty well, there is a real phenomenon there.