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Sampling only people who self-select themselves into one specific group (sperm donors) definitely isn’t a random sample. There are obviously correlations between income, geographic area and age with the desire to become a donor.

Don't sperm banks collect all of that relevant information? I would assume they collect at least age, income, and education. Probably some health history as well.

There can easily be a causal link. It’s a self selected group specifically on sexual reproductive strategy. I don’t need to be able to explain the relationship in detail; it’s enough that it’s plausible and that it hasn’t already been studied.

You also need to live somewhere where there are sperm banks. Lots of countries don't have them.