In 1961, a medical researcher named Michael Alpers began studying the reason why so many of the Fore people in Papua New Guinea were suffering from greater amounts of depression, emotional instability, and a loss of bodily functions among their women and children. The obvious answer seems to be in the Fore people's practice of funerary cannibalism, which is practiced as a way to preserve the skills and spirits of the loved one. The men of the tribe did not engage in the practice and didn't show the same effects.
It turns out that the act of eating brains exposes you to prions, which is essentially a normal protein gone bad. They no longer act like normal proteins and can change other proteins into prions. This creates an effect similar to mad cow disease, which occurs when cows eat processed food made with cow brains. Fortunately, later studies have shown that regular exposure to cannibalistic practice build a resistance to prion based disease. So if you're gonna eat people, eat them often.