Drawing on life history theory, Ellis and associates' (2009) recent across- and within-species analysis of ecological effects on reproductive development highlighted two fundamental dimensions of environmental variation and influence: harshness and unpredictability. To evaluate the unique contributions of these factors, the authors of present article examined data from a national sample 1364 mothers and their children participating in the NICHD Study of Early Child Care and Youth Development. Harshness was operationalized as income-to-needs ratio in the first 5 years of life; unpredictability was indexed by residential changes, paternal transitions, and parental job changes during this same period. Here the proposition was tested that these factors not only uniquely predict accelerated life-history strategy, operationalized in terms of sexual behavior at age 15, but that such effects are mediated by change over the early-childhood years in maternal depression and, thereby, observed maternal sensitivity in the early-elementary-school years. Structural equation modeling provided empirical support for Ellis et al.'s (2009) theorizing, calling attention once again to the contribution of evolutionary analysis to understanding contemporary human parenting and development. Implications of the findings for intervention are discussed. (Contains 2 tables, 2 figures and 2 footnotes.)