[r9etb]

But unattractive males know they are not going to bed so many females — so when they do get lucky they give it all they've got.

I won't argue with the data, but this "explanation" is ridiculous.

How do a guy's testicles "know" he's ugly?

[heartwood]

After a certain number, enough sperm is enough - unless there is competition.

My theory is that there are more and better quality sperm when there is likely to be sperm competition - when the man is ugly, the woman is attractive, or there is a new relationship, and the fastest, most numerous team has the best chance of winning.

There are some weird things that happen. For instance, at least one study showed that later-order births within a relationship tend to produce smaller children, as if the man somehow adjusts baby size to make it easier on the woman when she is a long-term investment.