&gt; There will always be pockets of population with high fertility,<p>What makes you say that? Is there some underlying principle I&#x27;ve failed to consider?<p>&gt; I have hard time picturing how that could lead to the complete extinction of mankind.<p>It&#x27;s simple math. Below-replacement fertility leads to extinction. More people die than are born, and since the original number is finite, the end number must be zero.<p>If you think that will not happen, what you&#x27;re imagining is that the fertility rate will either rise at some point in the future, or at least plateau at whatever number leads to a stable population.<p>Why would they rise? If you grow up in a world where everyone around you has few children, then you grow up thinking&#x2F;feeling that few children is the way things should be. You have few yourself. You might even have fewer than your parents&#x27; generation... but you&#x27;ll rarely if ever have more. On average, you&#x27;ll never have more.<p>Even in the fertility hot spots, this is true. They&#x27;re above replacement fertility still, but each generation has a lower number. It&#x27;s not exactly some sort of hidden trend, it&#x27;s pretty basic.<p>&gt; at the very least among highly religious communities.<p>It looks like the children of these communities break away from their religious upbringing at a higher rate than their pious siblings can replace with nephews and nieces. Especially in those religious communities that are unable to achieve a healthy economy... the FLDS communes that dispose of their sons, the ultraorthodox jews in their own self-imposed ghettos where there is little employment to be had.<p>&gt; Even if as you suggest reproductive behavior is transmitted,<p>That&#x27;s a little silly of me, actually. It must be the one behavior pattern that isn&#x27;t transmissible.<p>&gt; and simple natural selection will always, in the end, favor the reproductive behavior<p>The natural universe has no favorites. It does not preclude this sort of extinction. Those few who buck this trend will be starved for adequate mates at some point, and will raise their own children in a culture that ostracizes fertility...