CategoryScott Alexander [Astral Codex Ten]

• Is Sperm Count Declining? – People say it is, and Levine et al 2017 and 2022 find average sperm count has declined from 99 million sperm/ml to 47 million.
• Is Declining Sperm Count Really “Imperiling The Future Of The Human Race”? – Swan’s point is that if sperm counts get too low, presumably it will be hard to have babies. A graph shows fertility rate plateaus around 30 million sperm, and average ejaculation is 3ml, so total sperm count is 3x sperm/ml. If Levine’s linear model is correct, we have 10-20 years before median reaches plateau’s edge, and 10 years after that before it reaches zero.
• How Long Has This Been Going On? – The first recorded claim was Nelson & Bunge, 1974, and a few small studies suggest it may have started declining between 1951 and 1974.
• How Sure Are We That This Is Even Real? – Not too sure. Levine et al do good statistics, but the data is very noisy and there are many confounders (different types of people giving sperm samples, changes in ejaculation frequency, aging population, etc). Auger et al find six studies that are well done and five of them show declining sperm counts. Fisch and the Harvard Gender Science Laboratory paper point out the difficulty in collecting sperm samples and measuring sperm quality and make a few other points, but Auger’s is the best source.
• Did The Hypothesis Start By Mistake? – It seems like basically every person who proposed this hypothesis before Levine et al was mistaken, and had no right to conclude this from their small samples. Carlsen et al, the first to make it to the mainstream, should have found an increase, not a decrease, in sperm concentration.
• Where Is The Decline Most Pronounced? – Levine et al don’t compare developed vs. developing world counts, but unofficially it looks like sperm count is declining faster in the developed than in the developing world.
• Argued about it for the first forty years or so was working off of useless data, but by coincidence they happened to be right anyway – This is key to understanding the debate about declining sperm counts, as it shows how unreliable the data is.
• Where Is The Decline Most Pronounced? – Studies have found that the decline is pronounced in different regions around the world, and some countries are more affected than others.
• If Sperm Count Is Declining, What Could Be Causing This? – Possible causes of declining sperm count include plastics, pesticides, sunlight and circadian rhythm, diet and obesity, and porn.
• What About Animals? – Studies of farm animals bred through artificial insemination show inconsistent results of declining sperm count.
• Conclusions And Predictions – It is difficult to predict the future of sperm count, but some predictions include a consensus that sperm count is declining in 20 years, and that the most important factor is plastics or pesticides.

Published February 17, 2023
Visit Astral Codex Ten to read Scott Alexander’s original post Declining Sperm Count: Much More Than You Wanted To Know