How twin studies work. It's relatively simple.

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The comparison of identical and fraternal twins reveals insights into the genetic and environmental contributions to similarity in outcomes. A strong correlation between identical twins compared to fraternal twins suggests a genetic basis for traits. However, if the increase in genetic similarity does not significantly boost correlation, it indicates that environmental factors, or nurture, play a substantial role. Specifically, minimal increases in correlation despite genetic similarity point to the importance of parental influence and nurture in shaping individual outcomes.

Bryan Caplan of George Mason University and EconLog talks with EconTalk host Russ Roberts about the ideas in Caplan's new book, Selfish Reasons to Have More Kids. Caplan argues that parents spend too much time trying to influence how their kids will turn out as adults. Using research on twins and adopted children, Caplan argues that nature dominates nurture and that parents have little lasting influence on many aspects of their children's lives. He concludes that parents should spend less time and energy trying to influence their children. If parenting takes less time, then have more kids, says Caplan. The conversation concludes with a discussion of whether a larger population is bad for the planet.