I googled but could not find source. I only found more memes based on this graph. Which isn't to say it's fake, could simply be in some research paper hidden away under a paywall. 15k sample is a very big study though.

The graph alone is highly problematic to make any argument though as it doesn't specify other critical axis such as age which would highly change the meaning of this graph. The older you are, T levels fall, but your IQ increases. So age alone can potentially explain the phenomenon we're seeing here.

My attempt to find the graph only resulted in many counter points, however.

• https://pubmed.ncbi.nlm.nih.gov/9845018/ looked at only young men and women, so the age no longer becomes an issue. But among young men, higher T is advantageous. I can only read the abstract without paying. But does outline three conclusions there.
• https://pubmed.ncbi.nlm.nih.gov/15878571/ studied only 5 year old children. Which again showed a positive relationship between T and intelligence in boys.
• https://www.sciencedirect.com/science/article/abs/pii/030645309190018O this study on "normal" men and women, though not sure how they define normal, showed a negative relationship. But this once again doesn't define age in their study. Which continues with my previous point that simply older men will have lower T but higher IQ.
• Prenatal exposure to higher T also raised intelligence here: https://psychcentral.com/news/2011/03/14/testosterone-hormone-linked-to-higher-iq