Theories of ‘nature vs. nurture’ have been debated since before the turn of the nineteenth century, but professionals have yet to come to a consensus regarding the matter. Perhaps one of the best ways to study this phenomenon is by assessing and analyzing adoptees. If either nature or nurture were to have a distinguishably significant impact on character development, the changes would be most evident in individuals who were raised by people other than their biological parents, allowing for the separation of genetic and environmental factors in the evaluation of their impact.
Various studies have been done to analyze the role of adoption in child development, including studies regarding domestic and international adoption. Research groups such as van Londen et al. have demonstrated that the majority of internationally adopted infants are able to form familial bonds of secure attachment following adoption. In addition, adolescent and young adult adoptees have been studied regarding social performance and externalization of behavior following cognitive maturation. One of the longest studies analyzing the development of adopted children through young adulthood is the Colorado Adoption Project (CAP), which has longitudinally studied behavioral development of adoptees since 1975. In this study, there are two divisions of the analysis: Classic CAP, with subjects of ages 0-16, and Current CAP, of subjects of ages 17 until now. The studies of the CAP investigate
externalized effects on adopted subjects, such as relationships, milestones, and education.
While studies of adoptees in their behavioral tendencies have been done for a while, it was not until recently that studies were conducted regarding the physiological changes and variations in reflexive responses to stimuli. These types of studies arguably fit into the ‘nature’ category of the nature vs. nurture debate. Schoenmaker et al. explores the influence of adoption experience on basic physiological reflexes that are expected in young adults. The group looked at various aspects of normal maternal/paternal instincts of adoptees, including attachment security, perceived urgency, and respiratory sinus arrhythmia in the subjects. These variables were assessed with relation to a cry paradigm, designed by the researchers to evoke certain reactions from the participants. The most remarkable impact identified was the ‘secure’ and ‘insecure’ attachment classification of the individual. It was ultimately deemed that insecure attachment adoptees responded with significantly lower urgency to the cry paradigm than securely attached adoptees; furthermore, what can be described as a “higher tolerance” for infant distress, decreasing the likelihood of a response.
In order to understand what the above conclusions are suggesting, it is necessary to distinguish between secure and insecure attachment in individuals. Secure attachment, as outlined by Schoenmaker et al., secure attachment is rooted in a consistent base of support beginning in the early developmental ages, instilling the confidence that support will be there. In contrast, an insecurely attached individual is someone who lacked consistent and coherent support at a young age, and is therefore less motivated by personal relationships and less likely to support others. Based on these definitions, along with the adoptee status of participants, it can be argued that this is caused by nurture.
The difficulty in determining whether the differences between securely and insecurely attached adoptees stem from nature or nurture lies in the juxtaposition of the explanations as to why the distinction occurs. The potential difference in physiological changes based on attachment representation addresses the involvement of the parasympathetic nervous system in emotional and physiological response to attachment stimuli (ie, the cry paradigm). The decreased emotionality displayed by the insecurely attached individuals supports a distinction between secure and insecure adoptees.
Studies of adopted individuals such as this one performed by Shoenmaker et al. inform and suggest theories that could be useful to further research. However, the clinical analysis also adds another layer of complication to the nature vs. nurture debate. How can someone classify one way or the other when both mechanisms have such an influence on the individual? This article reinforces the battle between nature and nurture, which highlights the difficulty in coming up with a decisive explanation. Many people interpret lack of concreteness to be an indication of a marriage between the two theories. Regardless of the medical, evolutionary, and scientific advances that have been made in the past century, each study and article regarding human psychological development adds complexity to the ongoing debate of ‘nature vs. nurture’ – a debate that does not appear close to being resolved.
 Londen, W van (11/2007). "Attachment, cognitive, and motor development in adopted children: short-term outcomes after international adoption". Journal of pediatric psychology (0146-8693), 32 (10), p. 1249.
 Rhea, S.-A., Bricker, J. B., Wadsworth, S. J., & Corley, R. P. (2013). The Colorado Adoption Project. Twin Research and Human Genetics : The Official Journal of the International Society for Twin Studies, 16(1), 10.1017/thg.2012.109. doi:10.1017/thg.2012.109
 Schoenmaker, C., Huffmeijer, R., van Ijzedoorn, MH., Bakermans-Kranenburg, M.J., van den Dries, L., Linting, M., van der Voort, A., & Juffer, F. (2014). Attachment and physiological reactivity to infant crying in young adulthood: Dissociation between experiential and physiological arousal in insecure adoptees. Physiology & Behavior. pp. 549-556. Elsevier. *Main article on which this post was based