That is simply because individual in the same family are much more likely than unrelated individuals to share similar foundational experiences by virtue of their exposure to similar parenting and resources in their immediate environment throughout their early lives (Gerrig & Zimbardo, 2007; Rutter, 2006). Just as Rhesus monkeys tend to adopt maternal behaviors and elements of personalities of their mothers irrespective of their genetic inclinations, so do human infants and growing children and adolescents internalize and adopt various aspects of the behaviors and reactions exhibited by their parents and other significant adult behavioral role models in their lives (Gerrig & Zimbardo, 2007).
The quality of resources available to siblings (such as food, medicine, educational opportunities, etc.) is generally very similar within biological families (Gerrig & Zimbardo, 2007; Rutter, 2006). To the extent these factors contribute to the development of behavior, it is extremely difficult if not impossible to identify their connection to behavior in any way that distinguishes it from biological factors because siblings usually have access to identical or very similar resources as well (Gerrig & Zimbardo, 2007; Rutter, 2006).
In that regard, some of the most fascinating data into the dual influence of environment and genetics on human behavior has come from twin studies (Gerrig & Zimbardo, 2007; Rutter, 2006). Specifically, the fact that twins are sometime separated at birth and raised by completely different families in very different situations and circumstances provides a natural laboratory for inquiries into the relative influence of genetic predisposition as compared to that of external environment and experiences (Steen, 2006).
Consider that in empirical studies, researchers have established that among identical twins, the development of specific diseases, such as schizophrenia in particular, in one twin corresponds to a one-third probability of the same disease developing in the other twin (Steen, 2006). Likewise, the anecdotal evidence of identical twins is sometimes even more dramatic, such as exemplified by cases where twin brothers were separated at birth, raised in different families in different cities, only to meet by accident as adults and find out that they have chosen identical careers, recreational hobbies, and even married women with the same first name (Gerrig & Zimbardo, 2007).
The Complexity of Differentiating the Character of Difference
The prospect of differentiating or quantifying the undeniable influence of biological predestination and behavioral characteristics that are attributable to external environment is further complicated by the degree to which external environment and experience result in structural physiological changes, such as in the neural architecture of the human brain (Steen, 2006). A large volume of empirical studies has demonstrated the manner in which traumatic experiences, for one class of examples, permanently change subtle elements of the structure of specific regions of the brain and alter important aspects of hormonal and other strictly physiological responses thereafter (Gerrig & Zimbardo, 2007; Steen, 2006). Once those neural architecture and physiological responses develop, it is impossible to identify their source (i.e. whether they were originally functions of heredity or external environment) retrospectively (Steen, 2006).
Prior to the era of modern scientific methods and the technological research tools and technologies of contemporary researchers, it was understandable why the nature/nurture debate generated arguments that substantially disputed or at least discounted the influence of one another on human behavior. However, in much the same way that 20th-century physicists demonstrated that electrons are better represented as a “probability cloud” rather than by models depicting individual particles occupying specific points in space at a particular time, modern psychological methods have demonstrated that the distinction between nature and nurture involves a much more complex multi-dimensional analysis. Ultimately, the answer boils down more to probabilities than to absolutes, just as in the case of electrons within atomic structure.
Gerrig, R., and Zimbardo, P. (2007). Psychology and Life. Prentice Hall.
Rutter, M. (2006)..