a restricted condition, in which there were no colors or toys. The enriched condition had no effect on the maze bright rats, although it substantially improved the performance of the maze dull rats, such that there was no difference between the groups. Conversely, the restrictive environment did not affect the performance of the maze dull rats, but it substantially diminished the performance of the maze bright rats, again yielding no difference between the groups and demonstrating a powerful gene-environment interaction. A series of experiments conducted by Henderson on inbred strains of mice, in which environmental enrichment was manipulated, also provides evidence for gene-environment interaction on several behavioral tasks (Henderson 1970, 1972). These studies laid the foundation for many future studies, which collectively demonstrate that environmental variation can have considerable differential impact on outcome depending on the genetic make-up of the animal (Wahlsten et al. 2003). However, animal studies are not without their limitations. Gene-environment interaction effects detected in animal studies are still subject to the problem of scale (Mather & Jinks 1982), as discussed in greater detail later in this review.
