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Todo sistema, y por tanto cualquier sistema social, está sometido a un proceso constante de cambio a lo largo del tiempo. El estudio del desarrollo ya sea individual o social ha sido un campo plagado de problemas tanto teóricos como metodológicos. Una de las polémicas tradicionales en el estudio del desarrollo es la que se refiere a la naturaleza del mismo, esto es, si el cambio que se produce durante la ontogenia es de naturaleza continua o discontinua, con las implicaciones que esto tiene sobre el papel de las experiencias previas sobre el comportamiento posterior del individuo o sistema social del que se trate. El estudio de la continuidad en el desarrollo ha tenido como una importante fuente de datos el estudio de la conducta observable, infiriéndose a partir de los cambios en la misma, cambios en los procesos subyacentes. Ahora bien, la ausencia de isomorfismo entre conducta y procesos ha sido ampliamente documentada, lo que cuestiona la inferencia directa de discontinuidades en los procesos a partir de discontinuidades en la conducta observada. En este trabajo proponemos el uso de modelos de regresión jerárquicos "por piezas" como una herramienta de gran utilidad en el estudio de la relación entre discontinuidades conductuales y discontinuidades en los procesos subyacentes. Presentamos el modelo general y ejemplificamos su uso con datos de relación madre-cría en primates no humanos procedentes de un estudio longitudinal
It 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.
