Because relatively large brains are metabolically costly to develop and maintain, changes in brain size may be accompanied by compensatory changes in other expensive tissues.
Next time my wife asks me to clean the house I'll have to remind her that I already don't have enough energy to fuel both my brain and my sperm production.
I guess since I blow a load like a forgotten quart of chocolate chip ice cream left on the counter on a hot summer's day, I must fall into the other category.
I r teh stoopider.
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Some people are like slinkies, they aren't really good for anything, but they still bring a smile to your face when you push them down the stairs.
The average adult male human has sufficient blood to fuel his brain or engorge his *****. Alas, there is insufficient blood to allow both to function simultaneously.
Who needs scientists. I compiled that law when I was 19
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"I started out with nothin' and I still got most of it left" - Seasick Steve
China must be full of incredibly stupid people to have such a population. Justifying stupidity can be fun I guess, if it explains why the concept of contraception is lost on some people. (It's really an empty justification because no matter how much you run from it, stupid is as stupid does... *****.)