Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

No Two Alike

  • Carey FitzgeraldEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_2398-1



A book, written by Judith Rich Harris, which focuses on the theories and research that explain the individual differences in personality.


No Two Alike: Human Nature and Human Individuality is a book written by independent psychological scientist Judith Rich Harris and was published by W. W. Norton in 2006. The book – written in the theme of a detective story – investigates the potential causes of individual differences in personality as though they are suspects in a crime. Throughout this investigation, Harris cites research in many areas of psychology and biology – including traditional developmental psychology, twin studies, animal research, and behavioral genetics, among others. Harris’ investigation eventually leads her to present her own theory regarding the causes of individual differences in personality.

About the Book

No Two Alike begins by addressing many of the traditional and popular theories regarding why individuals differences in personality exist. These five theories – considered “red herrings” as they mislead researchers away from the true causes of individual differences – include the environment, heritability, gene-environment interactions, indirect genetic effects (also known as gene-environment correlations), and birth order. Although Harris does not fully discount these potential causes of individual differences, she cites important findings made in the areas of behavioral genetics and social psychology as evidence that these cannot fully account for all of the causes of individual differences across humans.

After eliminating those “red herrings,” Harris posits her own theoretical model that explains how and why individual differences in personality exist. This model contains three evolved psychological systems – each of which is distinct from the other. The first system – referred to as the relationship system – accounts for short-term behavioral differences based on interactions with the people in one’s environment. The relationship system allows individuals to store information regarding the people around them (e.g., who is kind, who is aggressive, who is trustworthy, etc.) allowing each individual to know how to act and respond when in the presence of each specific individual. This relationship system allows for variance in behavior but only accounts for short-term changes in behavior – as these behaviors only change when in the presence of different people.

The second system – known as the socialization system – is a product of socialization from many sources – parents, peers, etc. This system allows for individuals to develop an understanding of the social norms and accepted behaviors in their environments. This system – which supports Harris’ hypothesis from her previous book, The Nurture Assumption – illustrates how parents are not the only source of socialization for their children. Although children are socialized by their parents, they are also socialized by their peers – and peers continue to have a greater effect than parents’ as the children grow up. These various sources of socialization lead to differences in social behaviors based on what is deemed appropriate within each group. For example, an adolescent knows that s/he may engage in certain behaviors – such as swearing – in the presence of his/her friends, but that adolescent also knows that s/he should not – and therefore usually does not – swear when in the presence of his/her parents.

The third and final system – the status system – encourages competition with one’s peers. This system accounts for behavioral differences between individuals and their peers by examining peers as rivals for status and/or resources. The status system then collects knowledge of oneself and interacts with the socialization system to help elevate one’s social status over his/her peers. Harris argues that this system not only accounts for individual differences in long-term personality development, but it also accounts for individual personality differences between identical twins (i.e., identical twins may act differently from each other because they are engaging in different behaviors to achieve higher status than the other).


No Two Alike has been met with positive reviews by journalists and evolutionary psychologists (Kaighobadi and Shackelford 2008; Saletan 2006). It was written in a compelling detective-style format that is often viewed as fun and attention-grabbing. In this book, Harris provides evidence for rejecting some of the more traditional and popular theories regarding the causes of individual differences while also providing a new theory – rooted in evolutionary psychology – that adequately explains individual differences in personality.



  1. Harris, J. R. (2006). No two alike: Human nature and human individuality. New York: W.W. Norton.Google Scholar
  2. Kaighobadi, F., & Shackelford, T. K. (2008). Investigating the mystery of individuality. A review of Judith Rich Harris, No Two Alike: Human Nature and Human Individuality. Evolutionary Psychology, 6, 77–79.Google Scholar
  3. Saletan, W. (2006, March). Irreconcilable differences. The New York Times. Retrieved from: http://www.nytimes.com/2006/03/05/books/review/irreconcilable-differences.html.

Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.University of South Carolina – BeaufortBlufftonUSA

Section editors and affiliations

  • Carey Fitzgerald
    • 1
  1. 1.University of South Carolina - BeaufortBlufftonUSA