  This is a powerful model because it fits within a reductive framework (Fehr 2004). Genes cause anatomical differences, which cause hormonal differences, which cause brain differences, which cause behavioral differences. Birke argues that the relationship between hormones and behavior is much more complicated than this tidy reductionist picture implies. She advocates an interactionist model instead of the linear-hormonal model. She points out that research is beginning to embrace a much more complex picture of the causes of sexual behavior, a picture that involves interactions among the mother, the fetus, the physiological and social environment as well as the genes, the internal anatomy and the brain structure of the developing fetus. In this model, before birth the pups are influenced by factors such as the sex of other members of the litter, the mother's environment, and the mother's hormonal states. Birke writes, “Even before birth… it is difficult to separate the individual pup and its hormones from a network of complex processes” (Birke 1986, 97). After birth, factors such as the pup's own hormones, maternal care (which is differential depending on the sex of the pup), the physical environment and the other pups influence adult sexual behavior. When the pup becomes an adult, factors in its physical and social environment as well as its hormonal states and its own behavior influence its sexual behavior. The interactionist model does not ignore low level causes, such as genes or hormones, it simply refuses to privilege them over what may be thought of as higher level environmental and social causes of behavior. 