What do you think of when you think about family? Parents, children, spouses, or siblings? How about grandparents or cousins? For most people, relationships with those we think of as our family members are an essential part of our lives. As children, we were dependent on our families for our food and shelter. Our families protected us, loved us, and taught us about the world we were growing up in. Then, as young adults, many of us move away from our family circles, often to form families of our own. But many of us retain strong ties to our natal kin. Relationships with family can be important to our emotional health and can play a significant role in our social success. Family can be a source of great joy as well as great anguish. Our siblings, for example, can be our strongest allies and our most persistent opponents.

Our recently published Oxford Handbook of Evolutionary Family Psychology (Salmon & Shackelford, 2011) includes chapters by leading scholars, many of whom adopt an explicitly comparative and evolutionary approach to the family that focuses on the "whys"—the ultimate or distal reasons behind human, and animal, behavior. This perspective brings something to the table that is often ignored in the study of families. Humans are not the only species to have families. Many other species live in groups and have relationships with their parents, offspring, and siblings. Sometimes these relationships are short-lived, but sometimes they can last a long time and often appear to be characterized by strong bonds. Anyone who has read (or seen footage of) Jane Goodall's descriptions of the relationships between chimpanzee mother Flo and her offspring, especially Flint, the son she nursed until her own death, can not help but see a bond that many human mothers and children feel.

What is an evolutionary perspective on the family?

In many ways, early 20th century thinking about human behavior embraced Charles Darwin, or at least his functional approach to the study of life. To modern biologists and animal behaviorists, an evolutionary approach is second-nature. An adaptationist approach to animal families raises no eyebrows. But the last 75 years or so has seen an almost pathological avoidance of biology when it comes to the study of human behavior. We need to remember that people are just another type of animal, subject to the forces of natural selection just as all other species are. It's time to revisit the importance of our ancestral history and the selection pressures that built not only the organs of our body but also those of our mind. To explain this approach, we first provide a brief review of the process of natural selection, the special role of kinship in evolutionary analyses, and how adaptations can function as decision-makers, highlighting this with regard to kin relationships.

Natural Selection. When we refer to an adaptation, we are talking about an anatomical structure, physiological process, or behavior that made ancestral individuals' more likely to survive and reproduce in competition with other members of their species. Adaptations are shaped, or evolve, through natural selection. The process of natural selection is simply the differential production or survival of offspring by genetically different members of the population (Williams, 1966). If an individual (whether animal or human) is better able to survive and reproduce, they are more likely to leave offspring that share their traits (Darwin, 1859).

In other words, some feature of the environment poses a problem for an organism. Genetically based variants contribute to reproduction and survival with regard to that environmental condition. Individuals with those variants will be more successful, passing on their "good genes" and the resulting behavioral repertoire to their offspring.

Kinship Theory. Altruism has long been a topic of interest in the study of both human and animal behavior. To many early evolutionary thinkers, it was a puzzle. Why would individuals be willing to sacrifice anything for another individual? The logic of natural selection would seem to suggest that altruism should not exist and yet it is found throughout the natural world. Hamilton (1964) demonstrated that altruistic behavior (behavior performed at a cost to oneself for the benefit of another) could evolve if the individuals involved were genetically related. Even though the direct reproductive fitness of the donor is reduced, if his actions aid his own genetic kin, then he receives an indirect fitness benefit. At the time, this was a new way of thinking about fitness. No longer were organisms simply reproductive strategists (with fitness being measured in own offspring); now they were also nepotistic strategists (with fitness being measured in own reproductive success plus the reproductive success of kin). If an individual's genes are just as likely to be present in a sister as in a daughter, one would expect the evolution of sororal investment in the same way as one expects maternal investment.

For example, consider conflicts that occur within the family (animal or human). Hamilton (1964) pointed out that kin are valuable in the genetic sense (among others senses) and that what contributes to an individual's inclusive fitness also may contribute to the inclusive fitness of the individual's relatives. The more closely related are two individuals, the greater their shared genetic interests. But it is important to remember that although there is genetic commonality, there also are genetic differences and these can lead to conflicts of interest. These conflicts are often apparent when individuals are competing for the same scarce resources, such as mates, food, or social status. Trivers' (1974) analysis of within-family conflict made use of Hamilton's approach. Because the probability of an individual replicating its alleles through its own offspring is 0.5 (the degree of relatedness between parent and offspring) and through its full sibling's offspring (a niece or nephew) is only 0.25, natural selection will favor individuals that seek a greater share of their parent's resources. In other words, we expect a certain degree of sibling competition. In some species, this competition results in the elimination of a sibling competitor. In others, it just means a lot of headaches for the parents. From a parental perspective, they are equally related to all their children and grandchildren, so parents have typically been under selective pressure to resist a particular offspring's demands, especially when offspring are trying to extract more than their fair share of resources.

Consider the case of weaning conflict in a species that produces one offspring at a time. When an offspring is very young, parental fitness typically benefits most from investing highly in this current offspring. It's in the offspring's best interests to extract as many resources as possible up to a certain point, even at the expense of future siblings the mother could be having. At some point, the value to the offspring (who may be reaching a state of greater independence, able to obtain food on its own, etc.) of monopolizing such resources is outweighed by the costs in terms of its own inclusive fitness.

Typically, the mother reaches her own point of diminishing returns before the offspring does (after all, she is equally related to each of her offspring, whereas her current offspring is more closely related to itself than to future siblings). The period between when the mother's fitness is best served by decreasing investment in the current offspring and investing in future offspring (in reproducing again) and when the current offspring's fitness is also best served by the mother investing elsewhere is known in mammalian species as weaning conflict (see Trivers, 1974). The mother's fitness returns are decreasing but the offspring isn't quite ready to give up any investment. Conflict is most intense at such a stage. During this period, the mother's fitness is increased more by investing in additional offspring, whereas a particular offspring's fitness is increased more by continued maternal investment. The conflict ends as the fitness benefits of weaning shift to both mother and offspring (Drake, Fraser & Weary, 2008; Rehling & Trillmich, 2007).

Adaptations as Decision Makers. Adaptations can be anatomical, physiological, or behavioral. The beaks of Darwin's finches, often used to characterize the different species of finches living on the Galapagos, provide a classic example of an adaptation that is anatomical in nature (for review, see Grant & Grant, 2007). The dietary options available influenced the survival of birds of varying beak types and sizes, so that today we see finch beaks that are well suited to cracking large seeds in some areas, whereas finches in other environments have beaks that take advantage of other food sources such as insects. But adaptations can also be understood in terms of processes that carry out the cost-benefit analyses an ancestral organism required to survive environmental challenges. For example, the fever adaptation can be described as a set of decision processes for dealing with certain types of invading organisms. If you are being invaded by bacteria M, raise your body temperature by X degrees. The increase in body temperature may be enough to destroy the invader, which is beneficial to the individual (which is why when fever is prevented by drugs, resistance to infection is lower). But the adaptation is not cost-free. It takes a non-trivial amount of energy to raise body temperature. Perhaps more importantly, and especially in young children, the rise in body temperature can damage other systems if it is excessively high and prolonged in duration (Williams & Nesse, 1991).

From a decision-maker perspective, adaptations can be seen as decision rules or mental mechanisms designed by natural selection for producing the different behaviors required for ancestral survival, growth, and reproduction. Buss (1999) has suggested the term "evolved psychological mechanisms" for the specialized information-processing mechanisms that organize experiences into adaptively meaningful schemas. These mechanisms focus attention, organize perception and memory, and recruit specialized procedural knowledge that leads to domain appropriate inferences, judgments, and choices when activated by a relevant problem.

Kinship and Human Psychology

Anthropologists have long recognized the importance of kinship to the study of human social behavior, and one might have assumed the same of psychologists. It is true that attention to the family has been paid in areas such as developmental psychology and counseling psychology. However, it has been largely ignored in many other areas of psychology, including those areas in which its importance might have seemed obvious, such as social psychology (see Daly, Salmon, & Wilson (1997) for a discussion of the absence of the family in much of social psychology; see Burnstein, Crandall, and Kitayama (1994) and Michalski and Shackelford (2005) for examples of evolutionarily-informed social psychological research that takes kinship into account). A proper, evolutionarily-informed approach to a psychology of the family is by necessity a relationship-specific approach (Wilson & Daly, 1997). Humans, along with other species, have evolved specialized mechanisms for processing information and motivating behavior relevant to the specific demands of being a mate, father, mother, sibling, child, or grandparent. Kinship is not one relationship. It is many different relationships. The challenges that face mothers are different from those that face fathers or siblings.

Relationship-Specific Adaptations

Motherhood. There is no more essential mammalian relationship than that between mother and offspring. It should not be surprising, therefore, that it may be the relationship with the most specialized anatomical, physiological, and psychological mechanisms. The demands of motherhood go beyond conception, gestation, and nursing. Not all offspring are created equal. They are not all equally capable of transforming parental care and investment into the long-term success of parental genes. The result has been strong selection for the strategic allocation of maternal effort. The evolved motivational mechanisms that direct maternal investment decisions are sensitive to a number of offspring attributes, to the material and social situation, and the situation/condition of the mother herself (see Daly & Wilson, 1995, for a review).

However, mothers are not the only interested party. Offspring themselves have a role to play in shaping resource allocation. Parent-offspring conflict (Trivers, 1974) is a feature of sexually reproducing species because of the resultant genetic asymmetries in family relationships. A mother is equally related genetically to any two of her offspring, but each offspring is more closely related to itself than to a sibling (except in the case of identical twins). As a result, mother and offspring do not see eye to eye on the relative fitness value of other offspring or the allocation of maternal resources. This conflict over maternal resources provides an explanation for some puzzling aspects of mother-offspring interaction, such as the previously mentioned weaning conflict and maternal-fetal conflict (Haig, 1993; Trivers, 1974).

Fatherhood. There are significant similarities between paternal solicitude and maternal solicitude, but there also are several substantial differences. Parents have been selected to assess offspring quality and need, and for both fathers and mothers mechanisms motivating solicitude evolved to generate solicitude in relation to cues of the expected impact of any parental investment on the offspring's future success. Both father and mother have been selected to discriminate with respect to cues that the offspring is their genetic child. But it is true that for mammalian mothers the evidence is clear. If you gave birth to it, the baby is yours. For men, due to internal fertilization and relatively concealed ovulation, paternity is never certain (or wouldn't have been in our ancestral past). Putative fathers must depend on sources of information about the mother's likely fidelity, or the child's resemblance to his relatives or to himself. From this, one might predict that paternal affection will be influenced by paternal perceptions of resemblance. And, in fact, people do pay more attention to paternal resemblance than to maternal resemblance, with mothers and their relatives actively promoting perceptions of paternal resemblance (Daly & Wilson, 1982; Regalski & Gaulin, 1993).

Sibship. An evolutionary perspective also can generate insight into our understanding of sibling relations (Mock & Parker, 1996). Hamilton's (1964) analysis of the evolution of sociality and altruism in haplodiploid insects had at its core the shared genetic interests of sisters in such species. But although siblings, our close genetic kin, can be major allies they also can be our fiercest competitors, especially for parental resources. The result is sibling relationships that are often somewhat ambivalent across the lifespan.

Grandparenthood. Do we have adaptations designed specifically to deal with the problems faced by grandparental relationships? Or do these relationships merely co-opt adaptations for parenting? Postmenopausal women make significant contributions to the welfare of their grandchildren in many cultures (Lancaster & King, 1985; Sears, Mace, & McGregor, 2000). Thus, it is reasonable to suspect that mental processes specific to the allocation of grandparental investment may have been the targets of natural selection (Hawkes, O'Connell, Blurton Jones, Alvarez, & Charnov, 1998).

Euler and Weitzel (1996) examined the hypothesis that paternity certainty could influence grandparental investment (in addition to its impact on paternal investment) by asking adults to rate the degree of grandparental solicitude they experienced from each of their four grandparents. The results were striking, indicating a strong link between relatedness/paternity certainty and solicitude. Maternal grandmothers were rated the highest on solicitude, followed by maternal grandfathers, paternal grandmothers, and finally paternal grandfathers. From a theoretical perspective, a maternal grandmother has the greatest certainty of her grandchild's relatedness to her. A paternal grandfather faces a different dynamic. He endures two relationship links that can be broken by nonpaternity.

Kinship and Animal Psychology

When most people think about family, they think about their own relationships, their parents, siblings, and children, maybe cousins, aunts and uncles, and grandparents too. But we are not the only species that has families and for which kinship plays a significant role. Much of our understanding of human families and the mechanisms that influence family relationships was facilitated by the study of animal behavior and how animals invest in their own offspring (and sometimes in siblings or in the offspring of others). From work on the breeding success of birds, to the strategic allocation of reproductive effort to parenting or mating, to sibling competition and helpers at the nest, animal families not only are interesting in and of themselves, but also are important for the light they shed on human family relationships.

Animal Parenting. Consider the killdeer, a familiar bird in the grasslands of America. It is perhaps best known for its predator distraction display used to protect its chicks. If a predator gets close to its nest, it will attempt to lure the predator away with a display of vulnerability, behaving as though its wing is broken. The energy cost may be low but there is a risk (part of the bird's investment) that the predator will make the parent killdeer lunch (Brunton, 1990). How easy it is to appreciate such parental protection, seeing the connection between that and the protective behavior of human parents as well as many other animals.

Although many species of animal, particularly some aquatic ones, make no parental investment after spawning, many do invest in their offspring. Among mammals, it is typically the female who invests the most through provisioning and protection, although it is true that some males contribute significantly as well.

Because animals sometimes behave nepotisticly, engaging, for example, in cooperative grooming in Japanese macaques (Glick, Eaton, Johnson & Worlein, 1986) and in alarm-calling in Belding's ground squirrels (Sherman, 1977; alarm calling is done typically by females living near female kin), recognizing relatives has been a hot topic in animal family research. Do animals recognize each other as kin by frequency of contact? Phenotypic resemblance? Smell?

And like humans, not all family relationships are bright and rosy. Books on dealing with sibling rivalry between one's children abound, and in some animal species such sibling conflict is taken to Cain and Abel like heights with siblicide occurring in many species of birds. Siblicide in these cases has been interpreted as an adaptive strategy that benefits the surviving offspring and the parents, as grisly as this may seem (Mock, Drummond & Stinson, 1990; Mock & Parker 1997).

This article is intended to illustrate the many ways in which an evolutionary comparative perspective on the family can contribute to our understanding of behavior. We also hope that it provides insight into the kinship psychology of animals and how similar (and at times different) they are in comparison to our own kinship psychology. Much of the pleasure and pain of family life has been with us over the course of human evolutionary history. Our modern behavior is the product of our evolutionary response to the pressures of living as a social species just as the behavior of other social animals is a product of such pressures.

[Adapted from Salmon, C.A. & Shackelford, T.K. (2011). Toward an evolutionary psychology of the family. In C.A. Salmon & T.K.]

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