Doomsday theory?
Mankind’s numbers increase due to modern medicine.food production. The governments of the world realize that they MUST introduce mind control to a greater level than it is already at. Popluation increases theaten the enviorment and habbitat to such a degree, that extinction rates increase in an exponential rate. If the increase is not stoppered, the loss of Eco balance will hit a point of no return.
In secret, they start lacing food stuffs with chemicals to increase Oxytocin levels in peoples brains so that people are more compliant. Problem is, that the food stuffs are already so highly genetically modified, thing do not go as planned. Instead of compliance improvement, (So as to foster compliance to eco rules - such hard core conservation of pretty much everything) there is a massive drop in fertility in both men and women at an alarming rate. The pendulam swings way far the other way, and the extincion of mankind seems unavoidable.
The bio tech sector tries to take the logical next step, by modifing the human genome to increase feritily. They have limited sucess with some, only to find that the ones born have genetic mutations they did not expect. Both males and females are born with much higher androgen levels, which increase agression levels in both men and women. Soon, the new race is so violent that crime and chaos becomes rampant.
A great battle ensues between the older humans and the mutants. The mutans win in a great bloodbath. The bodies of the slain are so numerous, that there is no way to excape the deseases that become air born. Dysentery and a mutated strain of streprtococcus pneumoniea soon oblitorate the last of all man kind.