They didn't "choose" it. Their behavior malfunctioned because their conditions made them neurotic. And that behavior persisted even after it was no longer overc

They didn't "choose" it. Their behavior malfunctioned because their conditions made them neurotic. And that behavior persisted even after it was no longer overcrowded due to die-off, probably because of cultural transmission: females who grew to maturity in that environment conditioned what offspring they did have to have those same behaviors regardless of crowding or lack thereof. In the 1990s, I remember being bombarded with messages that "adoption was just as good as having your own child". In high school by teachers, when there was opportunity. From television and media. We've been conditioned too. I won't call it deliberate, but even if it were inadvertent would it matter? No words even have to be said, if you're a young child and they send you to school with other young children, and everyone you know is an only child, what do you grow up to think is the correct number of children for adults to have? The simple answer is "one", but there's an implicit "or zero" that should be tacked on. The children see these "zeros" too, when they notice the adults around them are childless. Those children don't grow up and choose not to any more than the rats chose not to.