This could be due to population size. Isn't there a species of fish where a male becomes a female when there are no other females left? It seems plausible that some mechanism is in place to self regulate.
North America has low population density relative to most of the world.
Probably even more so if you do some sort of aggregation of the density that people live at rather than averaging population across land area. Suburbia and such.
> North America has low population density relative to most of the world.
Well the quotient N(north america) / A(north america) is low, but this value does not have to match the average or median population density in areas around samples.
In other words, yes, America is a big country, but it's big cities are just as densely populated as big cities anywhere else.
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. Doesn't look like the meta analysis dealt with this.
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.
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.
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.
