A correlation between the sizes of male brains and their testicles have been theorised.
From the link below:
“Because relatively large brains are metabolically costly to develop and maintain, changes in brain size may be accompanied by compensatory changes in other expensive tissues,” wrote Dr Scott Pitnick in the journal Proceedings of the Royal Society B: Biological Letters.
I’d like to think that I’m intelligent and have a moderately sized brain. So therefore I must have small … *ouch*