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Ties that bind: From selfish genes to selfless behavior

By R. Michael Brown
Altruism—giving to others at a cost to oneself—is seen as a central problem for evolution theory, recognized as such by Charles Darwin. How can a behavior that advances the survival and reproduction of others, at the expense of one's own, be favored by natural selection? And, at a psychological level, what drives an individual to set short-term selfish (survival) interests aside and allocate valuable resources to others?

Recently, my daughter, Stephanie Brown, and I formulated a scientific theory—selective investment theory—that addresses these questions. Stephanie, intrigued by evolution since grade-school days, developed the foundation for the theory in the mid-1990s, while she was still a graduate student in social psychology at Arizona State University. I got involved a couple of years later, as she was transitioning from her role as a student to life as a research psychologist at the University of Michigan.

The particular form of altruism our theory attempts to explain is what we call costly investment, examples of which include parenting, allocating valuable resources to a spouse or partner, caring for a sick or injured family member or friend, and protecting comrades in times of conflict or war. These examples are so familiar to us that we may see no reason to question their evolutionary origin, or how they are accomplished on a day-to-day basis. But the costs they and other forms of altruism impose on the caregiver or protector can be substantial. Allocating resources to others takes time and energy, risks exploitation and, on occasion, places the giver’s survival in jeopardy. (Recall the nightmarish events this past April at Virginia Tech, when the student death toll might have gone even higher had it not been for heroic efforts like those of professor Liviu Librescu, a Holocaust survivor, who took a fatal bullet as he blocked the door to his classroom, keeping the shooter out and allowing students a chance to escape through the windows.)

Evolutionary Considerations

Instances of costly investment are observed in humans and many other social animals, so it is reasonable to speculate that such behavior has an evolutionary basis. But our ancestors would have been unable to transmit genes for high-cost giving to subsequent generations unless certain safeguards were in place. Our theory assumes that the most important of these safeguards is a condition we call “fitness interdependence”— the extent to which individuals have a stake in each other’s survival and reproductive success. Fitness interdependence makes altruists resistant to exploitation by the people they help.

We think that a bias to form social bonds with fitness-interdependent others constituted the evolutionary solution to adaptive problems posed by high-cost altruism. What is the justification for our argument? First, there is considerable evidence to suggest that social bonds in humans and other social species evoke emotional states that can motivate giving. Second, by definition, bonds are relatively durable, but perceptions of fitness interdependence may be fleeting, as when a growing child seems to need her parents less and less with increasing age and maturity. Yet, from an evolutionary perspective, it may be important for the parent to continue investing in the child in order to ensure her survival and reproductive success and, ultimately, the transmission of the parent’s genes to subsequent generations.

Psychological Considerations

Like others, we view the social bond as a connection between individuals that is relatively stable over time and across contexts. For each individual in a close relationship, the bond is a highly organized and dynamic memory structure that consists of stored thoughts and feelings about the other person, and about the relationship itself. Activation of the bond triggers events, such as release of the hormone oxytocin, that function to suppress emotions that promote self-interest, and stimulate feelings and thoughts that facilitate altruistic behavior.

Evidence for Selective Investment Theory

There are behavioral and neurophysiological data, from both nonhuman and human species, that are consistent with the central tenets of selective investment theory. And findings from recent survey and experimental research we have conducted with students at PLU and at the University of Michigan are consistent with the causal scenario implied by selective investment theory: that interdependence leads to bond formation, and properties of bonds (e.g., empathy) supply motivation to behave altruistically.

Perhaps some of the most compelling human anecdotal data come from observations of soldiers during wartime. For example, in one study, the most frequent response given by U.S. soldiers in Iraq when asked why they risk their lives in combat was “for my buddies.” As one soldier put it, “as far as myself, sir, I take my squad mates’ lives more important than my own.” And another soldier clearly recognized the importance of interdependence: “That person [fellow soldier] means more to you than anybody. You will die if he dies. That is why I think we protect each other in any situation.”


To our knowledge, selective investment theory is the first theory to highlight the social bond as a regulatory mechanism that evolved to override self-interest and motivate high-cost altruism. Indeed, our theory recasts the functional significance of social bonds, offering a striking contrast to traditional self-interest accounts of close relationships. These traditional accounts argue that we form and maintain bonds with others solely because of the benefits we receive from relationship partners (e.g., help, security, self-esteem, status), and view giving as a cost that must be offset by those benefits if the relationship is to survive. But through the lens of selective investment theory, the fabric of close relationships appears different. Giving, even sacrifice, is a prominent feature of healthy, enduring relationships with interdependent others.


It was a privilege and a delight to be able to collaborate with my daughter Stephanie on this project; it was also a lot of work. But the effort paid off last year when our formal written presentation of selective investment theory was published in a leading peer-reviewed journal, Psychological Inquiry. It is gratifying to learn that some leading relationship scientists and philosophers have already begun to incorporate our ideas into their own work. And it is satisfying to know that some of the questions raised by our theory have provided opportunities for psychology students at PLU and Michigan to become actively involved in the conduct of cutting-edge research, and to develop and hone research skills that may be important in their future endeavors.

Psychology professor Mike Brown has taught a variety of courses during his 25-year career at PLU, including evolutionary psychology, developmental psychology and statistics and research methods. In the past two years, he has co-authored with Stephanie Brown four articles published in refereed journals (Behavioral and Brain Sciences and Psychological Inquiry), a book chapter in Stephen Post’s 2007 edited volume, “Altruism and health: Perspectives from empirical research” (Oxford University Press), and several presentations at national and regional scientific conferences. Several PLU psychology students have either co-presented with Brown, or generated capstone projects of their own related to selective investment theory.

Photo top: Professor of psychology Mike Brown is a teacher, researcher and coordinator of the PLU Psychology Lab.