Suppression Field

The Combine, since gaining dominance on Earth, has imposed something known as a suppression field. The suppression field's main effect is to bring the human birth rate down to zero by prohibiting certain protein chains important to embryonic development from forming, effectively ensuring that the humans currently alive will be the last generation. Doctor Breen addresses this issue to the people via a broadcast during the early levels of Half-Life 2, as the response to a letter written to him by a "concerned citizen," and claims that the field will be deactivated once the humans prove they no longer need it. During the game, resistance members occasionally make humorous remarks such as: "When this is all over, I'm gonna mate," about the suppression field. It was initially thought that the suppression field somehow prevented human sexual urges, mainly due to Dr. Breen's statement that it was suppressing "the urge to reproduce".

In Half Life 2: Episode One, it has been revealed by Dr. Isaac Kleiner during his transmissions throughout the city, that with the near destruction of the Citadel, the suppression field has been disabled. He suggests that everyone "should do their part" in the re-population of the human species if they have escaped City 17, "before the Combine attempt to restore their dominion."