PIECEWISE HIERARCHICAL MODELLING AND THE ASSESSMENT OF THE RELATION BETWEEN DISCONTINUITY IN BEHAVIOURAL OUTCOMES AND DISCONTINUITY IN THE UNDERLYING PROCESS
AbstractIt is a well-established principle that behaviour and social relationships change over time. One major challenge of developmental behavioural scientists is to find quantitative models that can capture the nature of such changes, for example, if they are continuous. The issue of continuity in behavioural development is not an easy one to grasp both conceptually and methodologically. Abrupt changes in the rate of change or changes in the direction of change can be used as operational measures of developmental discontinuity. However, discontinuities in the development of behavioural outcomes may or may not reflect discontinuities in the underlying processes. In this paper, we use a two-piece linear model to explore its utility in the analysis of the relation between agedependent patterns of behavioural or relationship change and its underlying processes. Behavioural data from 23 motherinfant pairs of hamadryas baboons (Papio hamadryas), housed at the Madrid Zoo, were collected over the first year of life, using focal animal sampling and time sampling methods. The results of the multilevel hierarchical analysis revealed the existence of a curvilinear relation between the time that both partners spent together (i.e., within 50cm) and the infant’s age, the discontinuity in this behavioural outcome emerging at the age of about four months and a half. The application of the two-piece linear model showed that the association between the mother’s age and the developmental course of this spatial unit did change when the two periods (i.e., before and after the occurrence of the discontinuity) were analysed separately. This finding can be taken to suggest that the discontinuity in the behavioural outcome observed could be driven by a discontinuity in underlying processes linked to the mother’s age. We conclude that multilevel hierarchical models in general and piecewise linear models in particular can be helpful in unraveling principles of behavioural development.
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