According to recent studies, if the of the primate brain has a larger than average volume, the more likely that primate is to be sneaky.

I think the blank refers to 80% of the mass.  The theory called Machievellian Intelligence Theory, it states that the larger the brain of a primate the higher social skills it possesses. Aside from the complex skills, it also possesses better cognitive skills compared to other mammals.

11. gene12. allele13. crossing over14. linked15. gene16. x17. males, femalesapologies if numbers are out of order.