FTA: "The results, published in the journal Human Reproduction Update, showed a 52.4 percent decline in sperm concentration and a 59.3 percent decline in total sperm count among North American, European, Australian and New Zealand men."

> I think it's simple: fat unhealthy men have lower sperm counts and America has way more fat unhealthy men over the last 40 years.

While the US certainly has obesity epidemic, the results held across a number of countries in Europe and Oceania as well (and not in the African, Asian, and South American countries investigated; so the “Western” label is misleading, given that South America is as much part of the West as North America.)

I don't think obesity is the differentiating factor here.

We can only hope. This would be a great example of natural selection at work if fat unhealthy people couldn't reproduce and thinner healthy people could.

Only in our current environment. If food was scarce, wouldn't those who can store energy better be more likely to survive?

>If food was scarce, wouldn't those who can store energy better be more likely to survive?

it is also depends on how much more energy one has to expend to haul that stored energy around.

I get what you are saying, but I don't think being fat is evidence of being able to survive in an environment of food shortages.

Polycystic ovarian syndrome, which contributes to obesity in women, seems to have adaptive advantages in famine of the type speculated upthread (and also including increased fertility then, though decreased fertility in abundant food situations.)

It's not too much of a stretch to consider that there may be metabolic conditions in men linked to obesity risk that have the same kind of advantages in reduced-food-supply situations.

Does a 50% drop in sperm density result in a 50% drop in conception rates?

Since it's enough if a single sperm successfully makes the trip, that seems unlikely.

The situation is probably too complicated for a crude model like this to really apply well, but it seems to me the rough sketch of it should be more like playing Yahtzee and rolling for at least one six with a subset of the dice instead of all the dice. (In that case the chance to succeed is the complement of the product of all chances to fail)

I think this is a very good and relevant question. What does it actually mean for fertility? Quality of the sperm rather than quantity is probably a much more important factor for that. (What a shame that's much more difficult to assay...)

But regardless of whether it leads to a marked impact on fertility, for an important biological function to drop like this in so large populations is a big blaring air-raid siren of a signal that someone needs to sort out why and fix it.