Once a baby is born, the paramount question has traditionally been “Will it survive?” For most of human history the life cycle for many has been extremely brief- birth, followed quickly by death. Under primitive conditions it is not unusual for half of all newborns to die before the age of one, and prior to the Industrial Revolution even so favored a group as Europe’s royalty experienced infant mortality of over 200 per thousand live births. Now the U.S. rate is approaching 10 per thousand and even in families designated as living in poverty the rate is probably under 20. Widespread interest in infant mortality arises not only from its emotional impact but also because it is an indicator of economic and social conditions. The decline in the average rate has been accompanied by a considerable reduction in inequality of infant survival probabilities across groups, except for the black-white differential. The major proximate cause of the race differential and of infant mortality in general is low birth weight-that is, birth weight less than 2,500 grams (five pounds eight ounces), the standard set by the World Health Organization. Many observers ascribe differences in birth weight to income, education, and prenatal care, but as we will see, there are several reasons for doubting these explanations.