Until the past century, which life stage had the highest mortality rates?

According to the article Old Folks at Home, concerning the ability of older people to perform household tasks, the variables manipulated in the study included:

(a) safe or dangerous tasks (b) chosen or assigned tasks (c) easy or difficult tasks (d) tasks requiring, or not requiring, fast reaction time

An individual's sense of themselves as male or female is referred to as _______________

Fill in the blank(s) with correct word