The field of psychology remains a divided one. Several different sub-disciplines (e.g., developmental, cognitive, behaviorism, social, etc.) form what could be a unified scientific area. However, there is no widely accepted theory of unification. Charles Darwin once theorized that evolutionary theory would change the foundation of psychology; but over the years, evolutionary psychology has been met with hostile resistance from some of the prominent psychologists within the other sub-disciplines. Yet in recent years, all of the divided sub-disciplines of psychology have been slowly implementing evolutionary principles into their literature and research. This slow integration of evolutionary psychology into the other sub-disciplines indicates the possibility of a unified psychology with evolution as its foundation. This paper briefly reviews the literature within each major sub-discipline of psychology to show their implementation of evolutionary psychological theories, indicating the possibility of evolutionary psychology becoming the unifying paradigm upon which the entire field of psychology can be based. A call for action to continue this process is also discussed.

When the cost of altruism is low, individuals are more likely to help non-kin (i.e., friends and romantic partners) than kin. This trend is thought to reflect the fact that people tend to be emotionally closer with friends and romantic partners than kin. However, as the cost of altruism increases, altruistic preference shifts to kin. The present study highlights this phenomenon by examining altruism between siblings, romantic partners, romantic partners who have biological children together, and romantic partners who have adopted children together. Participants (n = 203) completed a questionnaire about altruism in low-, medium-, and high-cost situations. Participants gave more low-cost help to their romantic partners (regardless of whether they had a child together) than their siblings. More medium-cost help was given to romantic partners who had a child (biological and adopted) than siblings and romantic partners without children. In the high-cost condition, the estimated altruistic tendencies were stronger toward siblings and romantic partners who have a biological child than toward romantic partners with no children and partners with adopted children. Participants also believed they were more altruistic than their siblings and romantic partners.

Many studies on Hamilton's (1964) inclusive fitness theory have used the burning house and kidney donation examples of life-threatening altruism. However, these examples may not be sufficiently exhibiting the risk involved with life-threatening altruism that would have occurred in hunter-gatherer societies, such as fighting off attackers and/or predators. The present study examined participants' estimated likelihood to perform altruistic acts for specific kin members/friends in two violent life-threatening situations (i.e., being mugged and being chased) and two non-violent life-threatening situations (i.e., the burning house and kidney donation examples). Participants were 216 undergraduate students who completed a questionnaire on altruism toward an actual kin member/friend. Each questionnaire contained four life-or-death scenarios (two violent and two non-violent) in which either the participant's sibling, cousin, or best friend was in danger and needed help. Results indicated that people were more likely to help siblings than cousins and friends in both the violent and non-violent hypothetical scenarios. Participants indicated a greater likelihood to help people in violent situations than in non-violent situations. Women indicated a greater estimated likelihood than men to help people in non-violent situations while men indicated a greater estimated likelihood than women to help people in violent situations. Both male and female participants indicated a greater estimated likelihood to help women than men in violent situations.