Operational and conceptual confusion in life history research necessitates the two-tiered model.

Our systematic analysis of operationalizations and conceptualizations of harshness (extrinsic mortality) and unpredictability in the (psychology) life history literature highlights that (1) employment of extremely diverse measures contributes to the confusion about the effect of harshness on life history traits and (2) few measures reflect energetic stress or ambient EM, such that Ellis et al.'s two-tiered model should motivate future research.