Low fertility implies a “not low” referent; there are two. The first is “high fertility.” High fertility can be defined as some maximal, hypothetical level for a population, i.e., 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 ten births per woman, and even in the absence of substantial contraceptive use and abortion can be one-half 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).