Few would question the necessity of artificial limbs for amputees. But what of surgery to lengthen the legs of children who are merely shorter than average? Hardly anyone would challenge the decision to prescribe Aricept to people with dementia. But is it acceptable to give the same medication to airline pilots seeking sharper mental focus on long-haul flights?

Humans have engaged in biological self-improvement since long before recorded history, from the impotence-curing wild lotus brew of the ancient Egyptians to the herbal energy drink favored by early Olympians. Now biomedical...