Biological and Evolutionary factors that predict behavior

within family, clan, and tribe:

The Intention of this abstract is to present this theory in a form that is readable and understandable to the average person or non-scientist.  The ideas presented here are in no way intended to represent this work as exclusively my own. 

The research cited has been compiled from My personal studies as well as many other sources and behavioral scientists.


Three conceptual pillars for this Theory:

This framework for predicting behavior within a family or social order is based upon the integration of three overlapping Theories or concepts of evolutionary adaptation in social animals.

1. Ecological constraints, theory:
Examines factors affecting dispersal options of sexually maturing offspring. It specifies the conditions for delayed dispersal and, hence, the formation of family social units.
2. Kin selection, theory:
Helps identify the types and contexts of social interactions expected among family members (most of whom are genetic relatives).
3. Reproductive skew, theory:
Incorporates and expands on ecological and kinship to predict when conflict over reproduction is expected within families and when such conflict will result in reproductive sharing. Such sharing creates extended family structures and social orders such as clans and tribes.

This framework is further based upon observation of four parameters of behavior that affect reproductive success and long term adaptation and can be used to logically explain many aspects of family life in birds and mammals.

Four basic parameters for this Theory:

1. Genetic relatedness, or kinship: The value of protecting and preserving closely related animals.
2. Social dominance: The influence of polygynous behavior and dominant animals upon family stability, particularly in providing a conduit for genetic diversity by integrating non related animals into the group with less stress.
3. Benefits of group living: The value of specialization, commerce, and shared responsibility within a family, clan or tribal unit.
4. Disadvantages of independent reproduction: Comparing the success rate for non-social species and independently living animals with that of social and gregarious animals.

E. O. Wilson published his treatise, "Sociobiology: The New Synthesis " about two decades ago. In the introduction he states, "the social sciences, (Anthropology, Sociology, and Psychology) are the last branches of biology waiting to be included in the Modern Synthesis." He was referring to the untapped potential of Darwinian thinking to provide a conceptual framework for better understanding many aspects of human behavior. In the intervening years, a small but growing number of evolutionary anthropologists and psychologists has begun making inroads into "darwinizing" the social sciences. But their progress has been slow and has often been met with resistance. Two often-cited reasons for such resistance are a belief that the tremendous plasticity that exists in the expression of social behaviors renders applicability of biological (genetic) principles unlikely and the view that culture, rather than genes, is of overriding importance in mediating such behaviors in our own species. The first assumption seems to contradict the idea of an inherited or genetic predisposition to specific social behaviors, while the second argues that, even if present, genetic aspects of social behavior will be of minor importance.

Both criticisms can be easily dispelled. Recent research has clarified the relationship between conditionality and the evolution of behavior. Indeed, the last decade has seen a marked shift toward viewing organisms as "decision makers," selected to accurately assess the consequences of different behavioral options available to them and to express those behavioral variants that maximize their success. Depending upon social circumstances, the optimal choice of behaviors may vary. But, providing that inherited variation exists in the assessment algorithms underlying the choices, natural selection can fine-tune the decision rules and thereby bring them into the realm of Darwinian predictability. The second criticism pertains to the use of evolutionary models to predict the social behaviors of higher primates, particularly our own species. The obvious importance of cultural influences on our behavior does not negate the probability that we humans also possess a set of biologically based predispositions for interacting with one another.

Definition of Family:
There is no strict consensus among social scientists as to what constitutes a family but Sociologists typically stress functional aspects of the child rearing unit: families are groups of co-resident adults responsible for the production, socialization, and education of offspring. Anthropologists stress kinship and inter-generational aspects of families: families are kin groups through which descent lines can be traced and which consist, at a minimum, of parent(s) and unmarried offspring. In humans, preferential interactions occur between offspring and their parents, even after offspring have established households of their own and this holds true for many "lesser" species as well.

Family formation and the importance of delayed dispersal:
Typically, this occurs when offspring delay dispersal and continue to reside with one or both parents past the age of sexual maturity. The presence of a breeding male is not essential to the definition of a family. Rather the presence or absence of reproductive males forms the basis of a second partitioning into bi parental, vs. matrilineal, families. It is useful to further differentiate between intact families, those where the original breeders are still the reproducers, and replacement families where, because of death, divorce. or departure, a breeder has been replaced.
For simplicity, I exclude large assemblages in which parents and mature offspring reside but have no spatial territory of their own although many of the predictions should also apply to them.
Definitions, and Observations, of Biological families and extended social groups lead to 15 Predictions:

Prediction 1. Family groupings will be inherently unstable:
Families will form and expand when there is a shortage of acceptable reproductive opportunities for mature offspring, and they will diminish in size or dissolve (break up) as acceptable opportunities become available.

Prediction 2. Families that control high quality resources will be more stable than those with lower quality resources:
The greater the quality of the resources controlled by the family, the greater will be the incentive to remain at home to inherit them. Offspring will be more choosy in their dispersal decisions and inheritance of the dominant breeding position represents a common route to achieving status among many social species. The result tends to foster polygynous dynasties and stable family units that retain control of high-quality resources over multiple generations.

Family Dynamics, Kinship and Cooperation:
The social interactions that occur among family members differ from the dynamics of other types of social groupings because families are largely composed of genetic relatives. Hamilton's theory of kin selection revolutionized evolutionary thinking about social behavior by stressing that individuals contribute genetically to future generations both directly, through the production of their own offspring, and indirectly, through their positive effects upon the reproduction of relatives. Kinship is thus predicted to influence the types of behaviors exhibited within families.

Prediction 3. Assistance in rearing offspring (cooperative breeding) will be more prevalent in family groups than in otherwise comparable groups composed of non-relatives:
Cooperative breeding occurs when an adult member of a social group provides regular care to offspring that are not genetically its own. It typically involves provisioning and defending such offspring for a prolonged period of time. Assistance in the rearing of young appears to be the norm in genetically related groups. In contrast, such assistance is rare in non-familial species. Nearly 90% of all birds and mammals exhibit cooperative breeding, and fully 95% live in family groups of some kind.

Prediction 4. Assistance in rearing offspring will be expressed to the greatest extent between those family members that are the closest genetic relatives:
Most social birds and mammals demonstrate preferential allocation of aid based upon closeness of kin.

Prediction 5. Sexually related aggression will be less prevalent in family groups than in otherwise comparable sized groups composed of unrelated individuals:
Social species show strong tendencies to pair exogamously, as matings between close relatives often have damaging genetic consequences. Consequently, natural selection fosters adaptation of instinctive behaviors that preclude mating with close genetic kin. However, incest is certainly not unknown among vertebrates and if constraints on independent breeding become severe, a point may be reached where it is better to breed incestuously than to risk not breeding at all.

Prediction 6. Breeding males will invest less in offspring as the certainty of paternity decreases:
This prediction is not restricted to familial species but will apply to any situation where males help to provide care for dependent young. Parental nurturing is a form of kin assistance and offspring share half their genes with each biological parent. For most vertebrate species, parental uncertainty is primarily a male problem and males generally fail to care for offspring if genetic linkage is uncertain. Species that form extended clans or tribes of related and unrelated individuals, tend to have a polygynous dominant male that acts as a force to integrate and protect unrelated individuals from harm, thus assuring genetic diversity within the group.

Family Dynamics are disrupted after breeder loss or replacement:
The death, divorce, or departure of a breeding parent, and its replacement from outside the group will alter the basic genetic structure of the family unit. Such loss of a parent is expected to profoundly alter the social dynamics of the family group and, potentially, its stability as well.

Prediction 7. The loss of a breeding adult will result in family conflict over the filling of the resulting reproductive vacancy:
Family formation is a biological "solution" to the problem of a shortage of acceptable breeding vacancies and one route to becoming a breeder is to inherit the natal breeding position. The loss of a parent creates an excellent opportunity for an offspring (or another subordinate kin in the case of extended families) to assume breeding status without leaving it's home territory. Offspring of most species will not mate with a parent so competition for breeder status must involve an unrelated mate from outside the group. In simple families where only one female breeds, offspring must also prevent the remaining parent from reproducing and prevention can include resisting the settlement of a replacement mate, behaviorally suppressing breeding by the surviving parent, or physically evicting the parent from the group. In most vertebrate species, dominance is influenced by gender, age, and genetic predisposition to dominance. In general, males are dominant over females and older individuals are dominant over younger ones. Thus when a breeding female dies, a surviving daughter will rarely challenge her father for breeder status. However, challenges often arise between sons and mothers and sons and potential step-fathers after the loss of a dominant male breeder. in simple conjugal family species, sons replace their deceased fathers as breeders much more often than daughters replace their deceased mothers, but in complex tribal circumstances, loss of the dominant "Alpha" male can foster chaos and a clan or tribe may significantly restructure it's internal alliances before stability returns.

Prediction 8. Sexually related aggression will increase after the re-pairing of a parent:
If a divorced, widowed, or abandoned parent takes a new mate, the replacement is almost invariably from outside the family group. Parental re-pairing often provides a mating opportunity that is exempt from biological incest restrictions. Mature offspring of the same sex as the remaining parent can gain direct fitness by reproducing with the new unrelated breeder. In the case of simple conjugal families, the surviving parent and its mature offspring will now compete for sexual access to a replacement mate or step parent. This conflict may be especially severe when there is only a small variation in dominant tendencies between the surviving breeder and its offspring.

Prediction 9. Replacement mates (step-parents) will invest less in existing offspring than biological parents:
Replacement mates will be unrelated to current family members, therefore they gain nothing from helping to rear dependent offspring of their predecessor. If the replacement mate is of the dominant sex, they may abandon, evict, or even kill, immature offspring to increase the success of their own reproduction.

Prediction 10. Family members will reduce their investment in future offspring after the re-pairing of a parent:
Existing family members will be only one-half as related to future offspring produced by step parent and therefore will reduce their investment in future offspring following replacement of a genetic parent with a more distantly related or completely unrelated individual.

Prediction 11. Step-families will be inherently less stable than biologically intact families:
Genetic kin who remain attached to the family, loose benefits from helping rear young of the new breeding pair and also risk reduction of the family's investment in them by the replacement breeder. In simple bi-parental families, these factors tend to influence dispersal and family dissolution.

Complexity of Family and Shared breeding leads to Clans and Tribes:
The first Eleven predictions are most applicable to simple families, that is, Conjugal pairs, Individual parents and sexually mature offspring. In these simple families, reproduction is generally restricted to a single dominant female although two or more females of the same family may breed, giving rise to extended family structures. In vertebrates, this most typically occurs when offspring reproduce concurrently with their parent. These extended clans may be matrilineal or bi-parental. They generate assemblages of relatives that may include grandparents, aunts, uncles, cousins, in-laws and even unrelated animals who may align themselves with the group for protection.

The fundamental challenge in explaining the diversity of family structures lies in identifying the conditions under which reproduction becomes more complex and nurturing becomes a shared function.  The theory of how an extended family, clan or tribe is formed and maintained is commonly refereed to as reproductive skew.

Nearly 85% of all nominally social animals tend to form polygynous groups, clans, or tribes. The central idea of skew theory is that the dominant animals may, under certain circumstances, share reproduction in order to induce subordinates to remain with the group. The central behavior involves reproducing and nurturing offspring cooperatively by a clan or group.   Within these clans, tribes, or extended families, dominant individuals (typically males) control the reproduction of subordinates, and in many cases maximize their own genetic fitness by monopolizing breeding. (as in wolves)

This behavior tends to amplify genetic differences over generations and foster specific kinds of social interaction. Dominants become stronger while subordinate animals must adapt behavior that assures their own survival. Forming a bond or attachment to a clan or group is critical to survival and in many cases, subordinate animals do not breed at all, in deference to the Alpha. If subordinate animals are allowed to breed, a necessary precondition is that the Alpha male must benefit from his continued association with subordinates while sharing reproduction and realize a higher inclusive fitness than he would in the absence of the subordinates, the result is a "win-win" situation for the group.

Four additional parameters can be applied to predict conditions under which reproductive sharing may occur and the amount of sharing expected. The interaction of these four factors determines the outcome of reproductive competition within the family, clan, or tribe.

Four parameters for predicting Group stability:

1. The magnitude of any benefit realized by the dominant if the subordinate is induced to stay.
2. The expectation for success of the subordinate as an individual if it leaves.
3. The level of dominance or "pecking order" between the potential co-breeders.
4. The genetic closeness of the potential co-breeders.

Each parameter influences the relative payoffs of staying vs leaving for the participants. Collectively, they determine the leverage that the dominant has in "withholding," and the subordinate has in "demanding," a share of reproduction.

SKEW Theory forms the basis for the final four predictions:

Prediction 12. Less stressful living conditions will lead to increased mate sharing and cooperative reproduction:
As the severity of ecological constraints decreases, the likelihood of the subordinate's successful dispersal and independent reproduction increases. The leverage that a subordinate can wield depends both on the profitability of this dispersal option and on the benefit to the dominant of continued association with the subordinate.

Prediction 13. Shared breeding will increase when the difference in social dominance between potential co-breeders decreases:
A subordinate always has the option of directly challenging the dominant and a successful challenge can result in the overthrow of an "Alpha" and the subordinate's assumption of primary breeding status. However, costs of direct confrontation can be high for both participants as fights may lead to injury, eviction, or even death. The risk to the individuals and the group will be greatest when the disparity in their strength and other aspects of dominance is least. If an Alpha male perceives his challenger as "dangerous" it may then be advantageous to share reproduction. Sharing will increase the likelihood the subordinate will stay and continue to cooperate, thereby sustaining the advantages of group living.

Prediction 14. Shared reproduction within a family is more likely with siblings than with parents and grown offspring:
When same-sex siblings become co-breeders, the genetic relatedness between each sibling and the offspring of the other is the same. In contrast, mothers and daughters (or fathers and sons) are not equally related to one another's offspring. Therefore, the parent has more to gain from withholding shared reproduction than the offspring does from demanding it.

Prediction 15. Reproduction will be shared most with those family members to whom the dominant breeders are least closely related:
In species in which dominants actively suppress reproduction by subordinates, such suppression will be greatest in those subordinates to whom the dominant is most closely related. This counterintuitive prediction arises from the fact that subordinates indirectly benefit by remaining and helping in their family groups. All else being equal, these benefits are proportional to the subordinates' degree of genetic relatedness to the breeders. Subordinates to whom the dominant pair is most closely related receive the largest amount of indirect benefit from helping and therefore require the smallest "staying incentives" to keep them in the family group. Conversely, distantly or nonrelated subordinates gain little or no indirect benefits from staying and helping and, thus, require a greater amount of personal reproduction to be induced to remain.

Many aspects of social behavior, ranging from the formation and stability of family groupings, to the roles that different family members adopt in their interactions with one another, to the structural form of the family, clan, or tribal unit itself, can be understood within a Darwinian framework. This framework is based upon three major concepts of behavioral ecology: ecological constraints theory, inclusive fitness theory, and reproductive skew theory. Each assumes that individual behaviors are based, at least partially, on adaptive decision rules shaped by natural selection.
From these basic tenets of evolutionary biology, This set of 15 predictions that provide an integrated framework for considering the workings of the family and other kin associations. The stability of a family or group depends on the trade-offs between the fitness benefits available from staying home vs. those available from dispersing to attempt reproducing elsewhere. Delayed dispersal, coupled with the sharing of direct reproduction by dominants with subordinates, gives rise to a diversity of extended, as opposed to simple family structures. The behaviors of families, clans, tribes, and to an extent even the complex interactions of modern human cultures can be explained with these predictions.
Social vertebrates, and particularly birds, are excellent subjects for testing hypotheses about living in groups. Social behaviors, while complex enough to be inherently interesting, are simple in the sense of being relatively free of complex cultural influences. Variations in family structures and the tactics employed within the family or group, can be seen to have direct parallels in human society. Animal studies provide us with valuable insights about human social behavior. As we view the fundamental biological rules that govern social interactions, we can perceive the underlying non-cultural factors that affect our own social behaviors.

Human behavior, while highly flexible, is not infinitely malleable. There is no doubt that human behavior is strongly shaped by the cultural environment. But, Humans also possess a complex set of biologically based predispositions for interacting within small group social orders, a biological heritage based on flexible decision rules that were adaptive during our long evolutionary history of living in extended family groups. This view of family systems may seem to be in conflict with the views of psychologists and social scientists but the framework for predicting behavior presented here in no way contradicts any accepted theory of animal behavior. These predictions are offered as a tool to supplement studies of the proximal and developmental mechanisms that influence the behavior of human family members. The adaptive and mechanistic viewpoints are not mutually exclusive. Rather they constitute paradigms reflecting different levels of analysis. Each stands to benefit by incorporating elements of the other into its approach.


Stephen T. Emlen, PhD. Section of Neurobiology and Behavior, Cornell University, Ithaca, NY. Published an outline of the evolutionary framework for viewing the formation, stability, structure, and social dynamics of biological Families, Clans, Tribes, and extended Social Groups, in the August 1995, National Academy of Science, Vol. 92. pp. 8092-8099.