In the demographic literature on developing countries, studies of mortality perceptions are conspicuous by their absence. Perhaps it has been assumed that when mortality declines, the decline will be quickly recognized by individuals and will then influence their demographic decisions. The possibility of substantial lags and biases in risk perception, which cause individual perceptions to diverge from the changing empirical realities, has not been much considered. Yet studies in cognitive and social psychology indicate that individual mortality perceptions are likely to be diffuse and may well be biased upward in relation to the declining empirical risks. If individuals are poorly equipped to recognize mortality decline, so too may be social groups-social learning will not necessarily correct individual misapprehensions. This note discusses the perceptual limitations that may delay recognition of mortality decline and examines the implications for demographic behavior in three areas: modern health care, fertility control, and children’s schooling.