Low fertility implies a “not low” referent. There are two. The first is defined as a maximal, hypothetical level for a population, that is, one with early marriage, close birth spacing, and no attempt to control family size. Fifteen births per woman provides a reasonable estimate of mean maximal fertility. The highest observed levels of fertility are closer to 10 births per woman and, even in the absence of substantial contraceptive use and abortion, can be one-half of this level. Regardless, low fertility implies much lower levels – levels approximating two or fewer births per woman. The fertility transition refers to the societal shift from high to low fertility (and is produced by deliberate attempts to achieve small family sizes).