Thread by @lymanstoneky

Recently the question of what role biological changes may have in declining fertility has come up, not least because of the recent book "Count Down" which makes this argument very forcefully.

I generally avoid the biological fertility question because, well, I'm not a biologist!

And I have not read Count Down yet, though it's on my to-do list.

But a version of the argument appeared this week in Scientific American, and so I went and looked at the studies involved. The argument is here: scientificamerican.com/article/reprod…

Let's start with the claim that "miscarriage rates are rising."

That claim is based on this:
ncbi.nlm.nih.gov/pmc/articles/P…

It uses retrospective reporting of pregnancy outcomes for women in the National Survey of Family Growth 1995-2015. This is the right data source to go to for this question.

But the issue is "retrospective reporting."

Here's the data shown for rising miscarriages generally (Figure 2) vs. *early* miscarriages in Figure 3. Early miscarriages rose by about 10 percentage points. Total miscarriages by about 7.

Which means late miscarriages actually *declined*.

But.... is this data really valid?

Well, folks, one important thing that has happened since 1990 is pregnancy tests have gotten cheaper and easier to use, sales have risen, and they've gotten more accurate earlier in the pregnancy.

MAYBE rates of early miscarriage have risen 80% as the author's suggest.

Or.... maybe not. Maybe we just got better at *detecting* early pregnancies.

Note as well that comparing NSFG estimates of miscarriage to prospective longitudinal studies suggests NSFG misses about *half* of all pregnancy losses, and about *50-80%* of all abortions.

Extensive research on demographic surveys in other countries suggest cultural beliefs and attitudes and subsequent fertility outcomes alter the odds a given woman will report prior miscarriages and abortion.

I like data. I like finding stuff in data. I like striking graphs from data. And I often share data that is in an incomplete form because it's thought-provoking.

But the most this data suggests is that "something changed." Was it detection? Values and attitudes? Miscarriage?

We don't know! Maybe the share of true miscarriages captured by the NSFG changed over time! It wouldn't need to change by much to yield the observed trend.

Next paper: testicular cancer.

Did it really rise?

The linked paper is here:
ncbi.nlm.nih.gov/pmc/articles/P…

Basically, they show that age-adjusted rates of testicular cancer have risen. Here's the graph:

Wow! Bad stuff, right?

Well, I was able to get the actual by-age data for the US from 1975-2004, similar to what they have. Here it is, in cases per 100,000 men in the age bracket in question:

Key thing to notice here is that there WAS a rise!

From 1975 to 1990!

Since then, the rise as petered out. Now it's about flat.

And even that rise has the same problem: detection!

We've gotten *better at screening* for cancers! I have no way to adjust for this, but I notice that the authors also did not make any effort to adjust for changes in screening intensity either.

Arguing that rates of testicular cancer rose because there was an increase in *reports* of testicular cancer is, uh.... not very strong proof!

Especially when even that rise in reports occurred literally decades ago, and has more-or-less stopped since then.

Also, in 2004, you might wonder WHERE were testicular cancer rates highest?

US, UK, Norway, Denmark.... countries with *high* birth rates compared to the rich world.

Cancer rates were lowest in.... Spain, Belarus, Japan, China, Singapore.... LOW TFR places.

White men had like 4x the cancer rate as black men, yet have LOWER fertility rates. Wild how that works!

WIth the US, where were testicular cancer rates highest?

In UTAH, which has the nation's HIGHEST fertility!

Where were testicular cancer rates lowest?

Atlanta and Hawaii.

I won't get into their birth defects section as the data there is *extremely* spotty. Puberty timing is also a dicey measure, since we know that puberty timing in the 1950s-1980s when data begins may also be chemically altered and unusual.

(i.e. significant lead exposure begins in the 1930s, so you can't say if we're moving toward more/less "natural" puberty rates by comparing to data in the 1950s or 1960s)

Now then, let's get to TESTOSTERONE.

Is testosterone declining across generations?

The citation leads here, which is a study of mostly post-reproductive-age men: pubmed.ncbi.nlm.nih.gov/19396984/

But my preferred study is this one of men in Finland. It has more young men in it and is all-around just a higher quality study IMHO. eje.bioscientifica.com/view/journals/…

Crucially, the Finnish study ***controls for BMI*** and finds that falling testosterone ***is not related to*** rising BMI. This may seem irrelevant, but a popular theory is that men getting fatter and lazier is leading to lower testosterone.

Here's what the Finnish study shows for testosterone levels, without controls.

Evidently, testosterone levels declined across birth cohorts at least through those born 1910-1960. But at some ages, there's a RISE for recent cohorts.

Finnish men in their 30s born in the 1970s have HIGHER testosterone than those born in the 1960s.

Much of the decline appears to have occurred in the middle of the 20th century, with less clarity about *recent* declines.

Notably, the latest US data supports this.

In a study of men 15-39, testosterone levels declined from 1999 to 2011, but rose from 2011-2016. Graphed below: urologytimes.com/view/testoster…

Testosterone rates *fell*.

They are not *falling*.

That may seem like quibbling, but it turns out to be important! Because it helps us interpret whether low-T portends low total fertility rates.

Here's that graph of US testosterone again, but with a line for US total fertility rates added.

TFR rose while T was falling.... then fell while T was falling.... then fell while T was rising.

This isn't to say testosterone levels have *zero* influence.... but the evidence that the changes we've observed are major drivers of demography is extremely thin.

As an added note: there's tons of research on factors driving low T.

Want to know what can ABSOLUTELY TANK your testosterone levels?

Getting married and having kids.

Run through Google scholar on "effect of marriage on testosterone" or "effect of childbearing on testosterone" and you'll find dozens and dozens of high quality observational studies and a few longitudinal showing that married dads have the lowest T.

High testosterone is strongly (and cross-culturally in virtually all non-nomadic societies!) associated with men.... investing little time with their kids, avoiding marital commitment, and generally being crap dads.

Meanwhile, relatively lower T is associated with getting married, staying married, and having lots of strapping young lads running around the house.

This is weird because we also know that lower T is correlated with many measures of sperm quality and reproductive health!

But it turns out the *social* effects of being a raging high-T arse more than offset the *biological* effects of having slightly stronger swimmers.

I don't know a nice way to put it but: "HIGH T GOTTA LIFT BROS" are a demographic dead-end in societies where high parental investment is the norm.

As an aside, another BIG driver of low-T is.... STDs. Turns out sowing a whole lot of wild oats sometimes means when you finally settle down to your own farm you got nothin' left. journals.plos.org/plosone/articl…

By the way, in case it wasn't obvious already, one reason I don't talk about this issue a whole lot is I'm a right-wing moralizing religious extremist who thinks you're all a bunch of sinners who need to abandon the ways of the flesh and come to Jesus.

I'm being tongue in cheek there but yeah basically celibacy-then-monogamy with high commitment and high parental investment is not only what is good and right but also, apparently, high "reproductive fitness." Whodathunkit.

By the way, here's another study, and in fact a huge-n one, of Israel.

Testosterone levels have fallen DRAMATICALLY in Israel from 2006 to 2019. Here's average T among 20-40 year olds vs. TFR:
ncbi.nlm.nih.gov/pmc/articles/P…

So even high-fertility societies like Israel have falling testosterone levels, even in periods with RISING birth rates!

This strongly suggests low testosterone, for whatever effects it may have, is probably *not* dramatically reducing fertility.

Finally, sperm counts. The link is to this article, which is the most recent compelling meta-analysis to date demonstrating falling sperm counts: academic.oup.com/humupd/article…

SOrry, back to testosterone: this study shows that sexually active men had LOWER T than sexually inactive, i.e. men with lower T ARE MORE LIKELY TO HAVE SEX. ncbi.nlm.nih.gov/pmc/articles/P…

Nor is it the case that higher-TFR countries necessarily have higher T: Bangladeshi men who grow up in the UK have HIGHER T (but lower fertility) than Bangladeshi men who grow up in Bangladesh. everydayhealth.com/mens-health/wh…

Okay, so sperm!

There is soooooooo much research on stuff impacting sperm quality. That one study has been cited over 500 times just since 2017, mostly by other empirical studies investigating what stuff impacts your swimmers.

For exaple, all that stuff about "good fats"?

High fat diets maybe reduce male fertility: onlinelibrary.wiley.com/doi/abs/10.111…

But other studies disagree, and suggest men should eat nuts: academic.oup.com/ajcn/article/1…

But one of the leading theories of right now is one you won't see anywhere in the OP article.... sleep!

Men who sleep more have healthier sperm, and the hours you sleep predicts your odds of having kids in the future. sciencedirect.com/science/articl… sciencedirect.com/science/articl…

*Sleep disruptions* turn out to be one of the strongest predictors of male inability to conceive, and even impact sperm counts themselves.

But the collection of sperm data turns out to be tricky. Because you see, it's not just environmental factors like air pollution that influence sperm count.

Even *seasonality* matters. sciencedirect.com/science/articl…

Guess what folks?

Sperm counts fall in the summer and autumn. So if the timing of collection varies across time, you could get a fake trend!

But more to the point, sperm counts are highest in the parts of the year when ***night is longest***.

Folks.

It's sleep.

Get.

Good.

Sleep.

This is actually where I think exercise has a really important role to play. Inactivity often leads to poor sleep! Stable diurnality is takes work!

Now, look. I'm not saying chemical environment doesn't matter. In fact regular followers know I'm a HUGE proponent of the view that chemical environment matters! Particulates, lead, all sorts of air and water pollution, they've al been shown to impact fertility!

And it's possible that some of that impact operates through sperm counts or testosterone or what have you.

But.... while these factors *matter* and probably have *some non-trivial effect*, the effect sizes are almost certainly still too small to explain much of recent trends.

The reason TFR in Korea is so low is not that Korean men have all been poisoned. Israeli men don't have bigger families because of all-natural diets. Biological factors may matter, but they aren't the main driver of recent trends.

Fertility rates are low because people are *deciding not to have kids*. That's the story.

In this thread higher up I mentioned "Research suggesting miscarriage rates are misreported" in developing countries.

@Anne_red_head pointed me to some of her work I had NOT seen but which is an AWESOME review of this issue: dhsprogram.com/pubs/pdf/MR25/…