A reward is a consequence of an activity that encourages more of that activity.
A punishment, of course, is a consequence of an activity that encourages less of that activity.
Now a reward can be in the form of a monetary profit. The reason that a monetary profit works as a reward is because people like to make money, and the reason for that is that people who liked to make money consistently outbred and outlived people who didn’t. You can work out further links in the chain of causation yourself.
At any rate, most of what we think of as rewards are rewards because most individual humans will change their behavior to get more of them.
A reward can also work by causing more humans to exist who tend to behave in the rewarded way. This would obviously be a longer-term reward. But it does the same thing… encourages more of the rewarded activity to occur.
So having kids and raising them to adulthood is itself a long-term reward for whatever behaviors are handed down to them through example or heredity. Having kids and letting someone else raise them to adulthood is a long-term reward for whatever behaviors are handed down through heredity, and raising someone else’s kids is a reward for whatever behaviors are handed down through example.
Which means the “free-rider” problem that appears to obtain from parents not getting monetary rewards for raising kids is not as bad as it might seem. The reward is a slower one, acting over several generations instead of a few years. But it is there. And so is the punishment… if you don’t have kids, whatever behavior caused you not to have kids will not be handed down and will occur less in the future.
The real problem comes from just what behaviors are being rewarded and punished in this way…