Homo Sapiens is a raging success. According to the UN, human population recently hit eight billion and will exceed 8.5 billion by 2030. Since the 1960s, when the number first hit 3 billion, it has taken little more than a decade to cross each new billion-person milestone. The UN predicts humanity will peak at 10.4 billion during the 2080s.

That model rests on two assumptions. First, that global life expectancy will rise again after recovering from a Covid-induced drop. In the 1950s life expectancy averaged 47 years versus 72.8 years in 2019. Second, that fertility rates stay on a declining trend, but the pace of the decline does not accelerate. The average fertility rate globally was 2.3 births per woman in 2021 compared to 5 per woman in the 1950s.

The prospect of providing food and energy for 10 billion humans is daunting. There is no way to add 25% more people to the planet without further interrupting natural ecosystems. And with all due respect to climate activists, there is to date no evidence that emissions controls will shrink humanity’s carbon footprint enough to counteract that additional population.

What if humanity peaks somewhere short of 10 billion? There are reasons to suspect that the fertility and life expectancy trends baked into UN population estimates could change.

In the US, the fertility rate has already dropped to 1.7 children per woman, substantially below the 2.2 per woman required for a stable population. Without immigration, America is already shrinking.

Fertility is correlated with wealth, leading to a large variance between developed and developing countries. The drivers of this correlation are female education, workplace participation, and access to contraception. In this context, the declining birth rates recorded in many countries today represent a huge victory for female empowerment and rising living standards.

What could cause those declining trends in fertility to accelerate? One potential factor is a highly publicized drop in sperm count among adult males. Two meta-studies from 2017 and 2021 (the first summarizing American medical studies, the second using global studies) suggest the gamete count for an average male has declined to 47 million/ml, down by 50% since 1970.

Artificial insemination studies indicate pregnancy rates drop off rapidly when gamete count per 3 ml sample is below 30 million. This means that if the average male in 2021 clocks in at 150 million gametes, and if the rate of decline claimed by those meta-studies stays constant, the average male would fall below the efficacy threshold of 30 million in 10 years, and fertility would collapse completely in 20 years.

There are reasons to doubt these statistics. Although the statistical analysis was well documented, metastudies generally have a lousy track record for accuracy. In this case there is a suspiciously wide variation in the findings of the individual studies reviewed. One study out of eight reported gamete counts increasing over time, and a third of the studies found no significant change.

Lastly, there are inevitable concerns about sample selection and methodology. In the 1980s, the World Health Organization recommended standardized approaches for gamete studies. Men seeking fertility assistance, who may be disproportionately represented in some studies, can skew the data. So do populations where average age was increasing over the study period. Some of the discussed studies predated the guidelines, and others deviated from the protocols.

Even if male fertility isn’t declining, there is evidence that evolving mating habits could alter historical birth trends. According to the 2021 General Social Survey, more than a quarter of Americans had not had sex in the prior year. This included 30% of the men under age 30. In the 1990s, roughly half of adult Americans were having sex at least weekly. By 2021 that figure dipped to less than 40%, and those who were active reported lower frequency.

Depending on how hard you sheltered during the pandemic, and who with, this data can either seem shocking or utterly predictable. It was, however, the highest level of sexlessness in the history of the survey.

Covid-19 trashed a global trend toward greater longevity, helping reduce the US average by 1.8 years. The efficacy of the various political and public health responses across the world is debatable. That Covid accelerated mortality primarily among the elderly and those burdened with co-morbid infirmities is not. Not all pathogens are so selective, and some are even more cruel. Ebola kills indiscriminately. The Zika virus targets pregnant women, with devastating results for their offspring. Polio similarly targets the young.

The greatest lesson of the pandemic was that our highly mobile, global supply-chain-dependent lifestyle increases our vulnerability to biological pathogens. We will experience more pandemics, and there is a good chance they will be more lethal than Covid.

The average mortality rate for Ebola cases across all outbreaks is 50%. The total case rate mortality for Covid-19 is slightly under 1%. Even without an ecological extremist like the Marvel villain Thanos, it seems likely that microbial evolution will put a serious dent in the herd before we grow to 10 billion.

Fewer people would be good for the environment and the climate, but how would that impact human societies? The short answer is: badly.

Healthcare, pensions and most social entitlement programs are premised on intergenerational pooling. Funding them is predicated on the assumption that younger, healthier participants are always entering the pool to support prior generations as their needs expand. The scenario evolving across the globe today – falling birthrates accompanied by longer life expectancy – threatens the solvency of such programs.

Japan is already experiencing a shortage of young workers to support a burgeoning elderly population. In France, protesters took to the streets to protest President Macron’s effort to increase the minimum age for pension benefits. In America, Social Security and Medicare face solvency risks unless revenues are increased or benefits curtailed.

Young people today face a difficult conflict of interest as they mature. Their political values prioritize decarbonization and environmental protection. In that context, like Thanos, they see humans as the alpha sin that triggers all ecological problems. But if they skip childbearing to avoid contributing to those problems, they will accelerate the collapse of society’s safety net programs and undermine their own financial welfare.