Bryan Caplan is a Professor of Economics at George Mason University, the author of “Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think” and a father of twins. He writes in the Wall Street Journal:

But twin research has another far more amazing lesson: With a few exceptions, the effect of parenting on adult outcomes ranges from small to zero.Parents change kids in many ways; the catch is that the changes fade out as kids grow up.  By adulthood, identical twins aren’t slightly more similar than fraternal twins; they’re much more similar.  And when identical twins are raised apart, they’re often just as similar as they are when they’re raised together.

... The obvious lesson to draw is that parents should lighten up.  I call it “Serenity Parenting”: Parents need the serenity to accept the things they cannot change, the courage to change the things they can, and (thank you twin research) the wisdom to know the difference.

... Yet eventually I noticed that twin research had another, far less obvious lesson for parents: Have more kids. When you ask high-effort parents if they want another child, the thought often frightens them. They’re already tired and stressed from the kids they’ve got; how could they endure the sacrifices required to raise one more? I reversed this argument. Others’ belief in the power of nurture made them reluctant to have more kids.  My disbelief in the power of nurture, by the same logic, made me eager to have more kids.

Parents who don’t take twin research seriously are “overcharging” themselves for every child—not financially, but emotionally. The blatant lesson of twin research is to stop overcharging yourself. Its subtle lesson is to rethink the number of children you want to have.