Even a rational optimist is pessimistic about some things. Here's one: the gradual distortion of the human sex ratio by sex-selective abortion. A new essay by the demographer Nicholas Eberstadt concludes that "the practice has become so ruthlessly routine in many contemporary societies that it has impacted their very population structures." He finds "ample room for cautious pessimism" in the fact that this phenomenon is still very much on the increase.

For obscure reasons,...