I've lately been looking into this, and I wonder what people here think. Seems like a low hanging fruit for successful altruistic people (possibly with other traits as well, such as being healthy). Both for the direct impact of creating a... better child than (than the counterfactual one), and also for the indirect impact of normalizing the practice.
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HFEA says that most donors "create one or two families, with one or two children each". The legal maximum is 10 families.
"You’ll normally need to go to a fertility clinic once a week for between three and six months to make your donation."
https://www.hfea.gov.uk/donation/donors/donating-your-sperm/