Uncertainty exerts powerful influences on life history decisions. This has been demonstrated in experiments on nonhumans and in mathematical models. Studies of human populations are suggestive of the effects of uncertainty, but they rely on measures of environmental stress. In this paper, we derive a new measure of uncertainty, upsilon (υ), for use in non-experimental studies. We estimate its association with reproductive behaviors in a longitudinal panel sample of adolescents in the United States. Results show upsilon’s internal structure is consistent (...) with theoretical models of uncertainty. Its associations with reproductive outcomes are also consistent with theoretical predictions. Upsilon seems to have its largest effect on the timing of fertility—increasing the odds of early fertility by a factor of 7, net of the effects of control variables. We discuss our findings for the association between υ and the timing of reproductive effort as well as our future research on υ. (shrink)