Individuals differ in innumerable ways, some adaptive, some maladaptive, and some neutral. Personality theories, we argue, can profit from distinguishing among these importantly different types of individual variation. This article outlines a taxonomy of origins of individual differences—environmental sources of adaptive differences (e.g., early environmental calibration), heritable sources of adaptive differences (e.g., adaptive self-assessment of heritable qualities), nonadaptive sources of individual differences (e.g., incidental by-products of adaptive variation), and maladaptive sources of individual differences (e.g., epigenetic trauma). The second section outlines empirical procedures for confirming or falsifying the differing conceptions of individual differences, with a special focus on empirically distinguishing adaptive individual differences from those that are maladaptive or nonadaptive. The final section highlights the importance of individual differences for solving social adaptive problems.