Heinsohn is not concerned with the absolute size of populations, but rather with the share of teenagers and young men. If this share becomes too big compared to the total population, we are facing a youth bulge. The problem starts when families begin to produce three, four or more sons. This will cause the sons to fight over access to the positions in society that give power and prestige. Then you will have a lot of boys and young men running around filled with aggression and uncontrollable hormones. And then we shall experience mass killings, until a sufficient number of...