Speculation On The Blood-Brain Barrier

Does neuron function have a selection tradeoff with disease resistance?

Hypothesis:

The blood-brain barrier is largely the reason why intelligence is so polygenic.

Letting diseases kill brain tissue is a catastrophic failure condition, and biology has given us the solution of a blood-brain barrier where substances of sufficiently-tiny atomic size are all that can pass it. However, if cell function requires complex nutrients, then this means that the brain has a unique need to manufacture the essential nutrients that it lacks by means of the more elementary resources it has access to.

• Prediction: General cell function in the brain is lower than elsewhere in the body

• Prediction: Cell function in the brain is harmed more by mutation than is cell function elsewhere.

If brain infection, while more deadly, is survivable, and if the size of the filter varies according to genotype, this could mean an evolutionary tradeoff between intelligence and resistance against disease, as relaxing the blood-brain barrier for the sake of easy brain function will help one and harm the other.

Maybe this has something to do with the stereotype of the sickly genius?

Note: This is one of those things where it wouldn't be surprising if nobody's ever measured for individual differences in the size of the blood-brain barrier. However, a good first step is to look at the old research and see what confidence intervals they give for its size. While part of this will be measurement error, it may give a ballpark for the maximum amount that people can differ assuming they actually look at multiple people rather than measuring an individual person multiple times.