Personality or emotionality development in animals is influenced profoundly by experiences and stimuli in the early stages of growth. Previous work at Leeds University (Stedman and Varley, 1990) has shown that there are large variations in the personality characteristics of multiparous sows and these are stable and repeatable over time. The work also showed that personality traits may account for a significant proportion (17.8 %) in the overall variations in prolificacy in the pig.

The effect of different environments on individual animals may have its foundation in the perception of stress by each individual. What may be tolerable for one animal may not be so for another. Stress is a notoriously difficult concept to evaluate in terms of its effects on animals and on the production of animal products. The components of a high stress system will include; very high stocking densities, repeated new social grouping, barren environments, poor thermoregulation and human-animal interactions which cause either physical or psychological trauma to the animals. Reproductive processes are particularly affected by stress and some of the manifestations of this are: reduced litter size, failure to show oestrus, poor conception rates and sporadic puberty attainment.