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What causes interindividual variation in fitness? Evidence of heritability of latent individual fitness traits has resparked a debate about the causes of variation in life histories in populations: neutralism versus empirical adaptationism. This debate about the processes underlying observed variation pits neutral stochastic demographic processes against evolutionarily relevant differences among individual fitness traits. Advancing this debate requires careful consideration of differences among inference approaches used by proponents of each hypothesis. Here we draw parallels between several disciplines focusing on processes generating variation in individuals' life-course, and we contrast methodologies to disentangle these processes. We draw on other disciplines to clarify terminology, risks of flawed inference, and expand the panel of hypotheses and formalizations of processes generating variation in life histories. Trends Evidence of heritability of individual fitness traits in wild populations has reopened a debate about the relative contribution of neutral, stochastic demographic processes to observed variations in life histories. There are conceptual differences among published studies documenting heterogeneity in life histories; differences so fundamental that they led to misunderstandings between schools of thought. The question of the processes generating heterogeneity in longitudinal trajectories has stimulated a large body of work in econometrics, political, social and biomedical sciences, which have highlighted risks of flawed inference; these risks have been overlooked in biology. Other disciplines offer useful frameworks for future work on life histories in three areas: terminology, the characterization of the diversity of processes underlying variation in life histories, and the methods of statistical inference to disentangle these processes.