Researchers have been able to plot the respective likelihood of developing the disease among individuals with either contributing factor in comparison to individuals with both contributing factors, demonstrating that the combination of biological and environmental influences is a much stronger predictor of depression than either factor on its own (Gerrig Zimbardo, p.498).

The Significance of Extreme (Deviant) Human Behavior and Identical Twin Studies:

Some of the most dramatic evidence of the significance of external environment and experience on personality development comes in the form of the common experiences that criminologists and criminal psychological profilers have identified as playing a role in the evolution of profoundly deviant human behavior. That evidence is relevant by virtue of its extreme departure from 'normal" human behavior as well as the fact that the individuals involved typically share very specific childhood experiences but no apparent biological similarities.

Serial killers undoubtedly exhibit some of the most extreme forms of deviant social behavior, yet they share no known biological similarities that would predispose them to their pathological crimes. However, psychologists have identified very specific types of physical abuse and parental neglect in childhood, in addition to remarkably similar childhood experiences that consistently correspond to the eventual development of this particular form of criminally violent social deviance (Innes, p.7). With no identifiable biological factors to explain their extreme behavior, the implication is that it is exclusively a function of early childhood environmental experience.

By far, some of the most interesting evidence of the opposite end of the nature vs. nurture spectrum is represented by the phenomenon of identical human twins separated at birth. In principle, this provides the opportunity to examine specific traits that are strictly due to inherent biological factors. Whereas identical twins raised together share environmental influences as well their genes, identical twins reared apart share only their genes. Therefore, any behavioral similarities between them must be attributable exclusively to their...

They were reunited only because they both resided in the same metropolitan area after an acquaintance of one mistook the other for his twin. In addition to their interest in firefighting, both shared other behavioral similarities such as a preference for Chinese food and, incredibly, both of them were known by their friends to have a unique way of holding their favorite beer (Budweiser in both cases) with a pinky supporting the bottom of the can. They had even married women with the same first name (Hennelly-Kieth, 2005).
Conclusion:

Behavior is partly a function of inherent biological factors and partly a function of external conditioning and exposure to environment. In many cases, it is difficult to distinguish which contributes more to outward behavior. Experimental observation of primates with known biological predispositions exposed to specific environments suggests that the latter is more important than the former. On the other hand, data from identical human twins and studies of deviant human behavior suggest that biology nature) and environment (nurture) are equally responsible for behavior.

Ultimately, our psychological development is likely a function of the complex interrelationship between our genetic heritage and the many influential experiences to which we are exposed during our formative developmental stages.