This video may leave you wondering if some parts of personality are set very early indeed. Well, wonder no more. Science has actually already looked into the matter, and found a significant correlation between fetal testosterone levels and vocabulary, suggesting that fetal testosterone might be involved in shaping the neural mechanisms underlying communicative development. That is: the LOWER your testosterone, the greater your vocabulary. One study found that girls had a significantly larger vocabulary than boys at the ages of 18 and 24 months after birth. Their discovery replicated previous findings of a female advantage in language ability, but revealed this sex difference at the earliest point of development.