Infertility is not the leading issue when people are asked, 'why are you not having children?'. Until it is, I don't see how it can be more important than societal barriers.

Lower sperm count and lower desire to have children might have a common cause.

Lower desire might even be causing lower sperm counts.

Anyway, sperm counts are not outside the normal range. It was just that certain groups had higher than needed levels.

The same things that cause a higher average life expectation? Maybe the microplastics floating through the atmosphere everywhere? The constant stress about what is happening with the whole world instead of just your neighbor?

When you can't afford to have children due to low pay, sperm count is out of the equation.

What doesn't make sense to me is the people that are concerned about society "changing", the native population not having enough kids, we are being "overrun" with immigrants etc. Are the same people that refuse to vote for politicians advocating higher minimum wages, more benefits and fairer taxes.

It can't work both ways, either you want to crush the average person with low wages and taxes, or you want the native population to thrive.

No one is going to subject themselves to poverty to help balance the demographics for the privileged.

>Men today produce fewer sperm than in the past, and the sperm are less healthy. The question, then, is what could be causing this decline in fertility[0].

This has everything to do with monogamous relationships and marriages in most of the developed human world.

In the natural world, there are many factors at play that help ensure only the "healthiest" males conceive with females. Most factors are indirect, such as males physically fighting each other for a mate wherein the "fittest" male would more likely come out superior and successfully mate.

One direct factor that has been practically eradicated from humanity, however, is direct competition between sperm. Some animals and many plants are polygamous, wherein a female receives sperm (or other forms of genetic material as applicable) from multiple males. The sperm have to compete with each other to reach the egg first and conceive, this encourages males with the "healthiest" sperm to pass their genetic material onto the next generation.

Monogamous relationships and marriages as seen in humans remove this factor completely, the "fitness" of a given male human's sperm is irrelevant to conception because competition between sperm has ceased to exist. Both unfit and fit sperm alike can conceive, assuming other indirect factors at play allow for it. Indirect factors that care not for the "fitness" of sperm.

Sure, maybe a percentage or two could be explained by this, but 60% in 50 years? Natural selection takes millions of years to accomplish a feat like that.

Humans may naturally be monogamous, at least to a degree. It is a common occurrence in the animal kingdom, in many bird species for example. Monogamosity is therefore a very unlikely candidate for fertility drop.

Apes are mostly polygamous though. And even if polygamy wasn't the norm culturally, the results of procreation have been polygamous. For example this study analyzed dna and shows how 8,000 Years Ago, 17 Women Reproduced for Every One Man.

https://psmag.com/environment/17-to-1-reproductive-success

This seems to be based on the idea that the only feedback mechanism for improving sperm is to have sperm fight with other sperm. That's not correct though. Whatever trait in sperm leads to pregnancy more often will be evolutionarily selected for. There doesn't need to be any direct fight between sperm.

The basis for how "healthy" sperm is is implied to be how mobile it is, and in an environment where sperm are competing to reach the egg first, being more mobile is absolutely a trait that will be evolutionarily selected for because less mobile ("less healthy") sperm will conceive less.

But in an environment where that doesn't matter anymore, it's only natural that sperm "health" will consequently deterioriate. Natural selection doesn't select for "the best", it only selects for "good enough". And if "unhealthy" sperm that can't move sufficently is "good enough", well so be it. Especially if we take factors like in-vitro and artificial insemination into account, those really remove the "health" of sperm from the equation of pregnancy.

If health has no effect on conception success, I don't see any reason to be worried about declining health.

I agree that in-vitro and artificial insemination could remove an important feedback mechanism for sperm evolution, and that would be a cause for worry.

My understanding is that it is starting to (presumably anyway) have an effect on conception, because a sperm that can't move is a sperm that can't fertilize an egg, and there aren't competing superior sperm to make up for the shortfall.

Which by itself is fine, really. Sperm that fails to reach an egg means the genes that made that sperm do not "deserve" to reproduce. However, if people want to make a fuss over low birth rates, and this is one factor behind it, it's going to get attention.

Monogamy has been around for a long time. I don't see how it could cause a drastic change in the last 60 years.

That's an elaborate portrait and narrative you've painted there. Do you have sources for that?

It was something I saw many years ago on NHK as one theory of why human sperm quality keeps deteriorating. As far as I'm concerned, it makes sense to me. Remove more and more factors that would encourage healthier sperm and it's only natural that our sperm will deteriorate.

As others have said, the decline is much too rapid in the last couple of decades for it to be explained by evolution. It sounds more like a theory spouted by chemical-/food-/etc companies who worry about getting the blame.