Explanations relying on complex gene-environment dynamics should be weighed against more parsimonious theories that postulate primarily direct genetic and environmental effects. For example, SPMs argue that people accumulate environmental experiences relevant for their personal identity with age, and these experiences have a causal impact on personality development. As these unique experiences add up over development, genetically identical individuals come to resemble each other less and less. In the context of a behavioral genetic model, this process means that heritability would decrease with age and environmentality would increase. Genetic effects may not be perfectly stable from birth because the accumulating environmental experiences act as a social push and thus constrain phenotypic expression to a certain extent. As individuals increasingly create a stable identity or environmental niche, they encounter less novel environments (i.e., increasing environmental stability), and this mechanism also acts to reduce novel social pushes (i.e., increasing genetic stability).
