Back in the 2000s, there was some interest in semen quality and intelligence:
Arden, R., Gottfredson, L. S., Miller, G., & Pierce, A. (2009). Intelligence and semen quality are positively correlated. Intelligence, 37(3), 277-282.
Pierce, A., Miller, G., Arden, R., & Gottfredson, L. S. (2009). Why is intelligence correlated with semen quality? Biochemical pathways common to sperm and neuron function and their vulnerability to pleiotropic mutations: Biochemical pathways common to sperm and neuron function and their vulnerability to pleiotropic mutations. Communicative & Integrative Biology, 2(5), 385-387.
Using the Vietnam Experience Study (US military veterans aged ca. 38), one of the only datasets that ever measured these two phenotypes together, they found correlations like these:
As far as I can tell, there is only one replication of this and it’s n=41 and didn’t find anything.
Since I have this dataset, I decided to look again. Arden et al did not show the scatterplot, but they should have because it looks like this:
This is the full sample, no adjustments or exclusions aside from missing data. The correlation is there, 0.14, vs. their 0.19, but if you look at the deciles on the right, you can see that the signal is entirely in the bottom 10% of men. This is more or less the case for every sperm metric that shows any signal:
Some of these metrics are interdependent since normal + small + large + tapered + amorphous = 100%. Thus, the simplest way to analyze that is just to use normal%.
So is this a real thing? Maybe the bottom 5-10% of men really do have quite a bit worse semen quality, or maybe there is some problem. Perhaps they were drunk at or before the testing, so their cognitive scores are bad and so is their semen quality. Or perhaps they didn’t stick to the rule of abstinence before ejaculation day. But before that, let’s examine the semen measures + covariates:
We can see moderate correlations among the semen metrics in an expected way. Factor analysis also confirms a 2-factor solution for volume and quality, so I will use those going forward. For good measure, here are the decile plots for semen quality and volume:
After looking closely at the 5% bins (not shown here), there is only an apparent effect in the bottom 10% of men sorted by g (intelligence), so I will use a binary for this group in the models. Here are some model results:
The results are pretty interesting. The bottom 10% men by intelligence have about 0.85 standard deviations worse semen quality, but only 0.35 SD less volume (p < 5% only). The quality link does not go away even with extensive controls, while the volume link became weaker and non-significant. The bottom 10% is only 48 men, so there is a limit to how much statistical precision we have to work with here, and we can’t say for sure about the volume. Note that Blacks have less semen volume than Whites, as may be expected by their smaller testicles -- a finding we previously reported from this dataset. Nothing predicts semen quality as well as low intelligence, though strangely abstinence predicts it mildly, perhaps not a biological link but a behavioral. A crude count of medical history (lifetime diagnosed conditions) predicts it slightly perhaps, while smoking, drinking, and BMI had no detectable effect. Concerning volume, abstinence is a decent predictor.
So it seems somewhat safe to say that intelligence does predict semen quality, but apparently only because the bottom 10% have really terrible quality. This is less easy to explain than positing some general fitness factor that underlies both, as done originally.