 Dear students, in this topic, we shall discuss the role of genes and environment in the development of behavior of an animal. The genes of an individual provide the instructions for the development of behavior. However, the expression of genes is influenced by many environmental factors. So, an animal's behavior develops by a complex interaction between genetic and environmental factors. Now we shall discuss the environmental factors involved in behavior development. For example, the environment of fertilized egg and animal's diet, animal's social interactions from which it can gain experience or learn something, the environment's conditions in animal's surrounding, temperature, water availability, food availability. These factors influence the development of behaviors. The behavioral ecologists or ethologists use two methods to determine the extent of environment's effect on the expression of behavioral genes. These two methods include cross fostering study and twin studies. In cross fostering studies, young ones of one species are nurtured by the adults of another species. As a result, the changes in the offspring's behavior are determined. The extent of these changes is a measure of how physical environment influences behavior. In cross fostering studies, some mouse species have behavioral differences that suit them well. Dear students, Twin studies are also carried out to determine the effect of genes and environment on behavioral development. Behaviors of identical twins are compared which are raised apart with those twins which are raised together in the same household. Twin studies are carried out on human and human's traits such as schizophrenia, anxiety disorders and alcoholism. When twin studies are carried out, they reveal the effect of different environments on the expression of the disorder. In the development of these disorders in humans proves the contribution of both environmental and genetic factors.