He gives three basic reasons why people should have more kids:
1. In happiness research, the big negative hit comes from having one child. Once you've passed that, the extra unhappiness from having more is fairly small.
2. Parents have little to no influence on how their children turn out, so you can relax, not bother to read to your extra kids or make costly "investments" on their behalf.
3. The more kids you have, the more grandkids you might get and everyone knows grandkids are *awesome*!
NB: I am not making any of this up!