Linked to the ACX post "Declining Sperm Count: Much More Than You Wanted To Know". If in 20 years, a brief review of the evidence convinces me that the sperm count decline was substantially real, I will resolve this YES. This is true even if the decline has stopped by 2043 (ie scientists then believe it was declining now, but is no longer declining at that time).
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@nsokolsky If environmental factors cause sperm counts to decline by (say) 50%, it seems unlikely that natural selection could double sperm counts in only 1–2 generations, especially since there are still ways to conceive even with a lower sperm count (e..g, by in vitro fertilization, or by having twice as much sex).
@VoyagerRock I think the panic is not about getting someone pregnant as soon as possible. It is about the fact that a lower sperm count sometimes cannot lead to a pregnancy at all.
@VoyagerRock yeah but what I want to get someone else's wife pregnant? If we're having an illicit affair there might not be enough time for that many orgasms.
@VoyagerRock To add to @HarishGanesan's point, sperm counts fall on a distribution. It's not just a matter of needing more sperm to get pregnant. A certain concentration of sperm per insemination is probably necessary. Given the existence of some threshold below which conception is difficult or impossible, a plenary lowering of sperm counts in everybody will necessarily move some people from being above that threshold to below it.
@ML Someone in the comments of the ACX post asserted that age was the primary cause and Scott said "Everyone realizes this and has controlled for it." https://astralcodexten.substack.com/p/declining-sperm-count-much-more-than/comment/12870524
This suggests it won't count as a "substantially real" decline if the full explanation is "more people are old".