Human biology is the study of the dynamics of biological variation in human populations. Of the numerous research avenues followed by human biologists one of the most challenging is the description and analysis of the relationships between the environmental milieux and the biobehavioural structures of human groups. The challenge lies not only in the specification and measurement of biological and environmental variables, but also the analysis of their interactions with each other within the contexts of hypothesis testing and the construction of models. Neither the biological states of populations and organisms, nor the surrounding environment, are constant, and it is this dual lack of constancy that creates problems of methodology and interpretation in research.

Seasonality, defined by Huss–Ashmore (1988a, p. 5) as ‘regular, recurring intra-annual fluctuation’ is of particular significance to human biologists when analyzing the effects of the environment. The individual, the population, and the environment all may fluctuate. And their fluctuations may be stable, i.e., predictable (see Bloom, 1964), or stochastic and predictable only within ranges of probabilities.

The appreciation of seasonality as a significant source of human variability has a deep basis within the history of our discipline. In 1810, Samuel Stanhope Smith, an American clergyman, published a landmark volume on the effects of the environment on human biological variability. In this work, Smith noted that: