Why are life trajectories difficult to predict? We investigated this question through indepth qualitative interviews with 40 families sampled from a multi-decade longitudinal study.
Our sampling and interviewing process were informed by the earlier efforts of hundreds of
researchers to predict life outcomes for participants in this study. The qualitative evidence
we uncovered in these interviews combined with a well-known mathematical decomposition of
prediction error helps us identify some origins of unpredictability and create a new conceptual
framework. Our specific evidence and our more general framework suggest that unpredictability
should be expected in many life trajectory prediction tasks, even in the presence of complex
algorithms and large datasets. Our work also provides a foundation for future empirical and
theoretical work on unpredictability in human lives.