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Abstract
Notwithstanding their relatedness, siblings vary as much as strangers with respect to personality traits. Attempting to explain this paradox across many publications, Frank J. Sulloway invokes evolutionary theory, specifically emphasizing Malthusian competition and referencing the concept of adaptive radiation, which produced beak variation among Darwinian finches as they spread across the Galapagos Archipelago. However, Sulloway understands birth order and other familial dynamics to create personality variation among siblings, using evolutionary concepts only as illustrative comparisons. The present paper argues that Sulloway mistook a literal truth for an analogy. Sibling personality variation does not mirror a process of evolution, it is a process of evolution. Substituting the macroevolutionary process of adaptive radiation for the microevolutionary process of adaptive diversification, and emphasizing the perpetuation of genetic material above the survival of the organism, sibling personality variation is herein explained as a hedge against lineage extinction. Unable either to predict environmental challenges or create pluripotent offspring, parents diversify their brood and thereby diversify their risk. As discussed, sibling personality variation as an ontogenetic process of adaptation remains relevant, but only in so far as it augments a primary genetic process of evolution.
Public Interest Statement
Across many traits, siblings are more alike than different. For instance, among brothers and sisters, height, intelligence, and coloration, correlate, whereas personality contrasts. As per standing theories, in an attempt to limit competition, siblings develop traits exactly opposite one another. With different personalities, they occupy different niches. This explanation is intuitive, but it has limited empirical support. In addition, sibling personality variation is substantially genetic and found among a variety of animals. The present paper argues that environmental factors like birth order, only accentuate preexisting genetically inherited differences between siblings on personality traits. Genetic variation among offspring across personality traits is an evolved hedge against lineage extinction. Genetic material more reliably passes from parent to offspring because of the behavioral variation that personality imparts. Personality traits impose costs and provide benefits at each level and so having offspring with different personalities diversifies risk. In contrast homogeneity within offspring personality would be akin to putting all eggs in one basket.
Competing Interests
The author declare no competing interest.
Notes
1. Malthusian is the adjectival use of the sir name Malthus, as in Thomas Robert Malthus, the eighteenth century divine and demographer. Malthus is best known for juxtaposing arithmetic increases in resources with geometric increases in human population. From this mismatch, dire consequences were predicted.
2. Inclusive fitness is also known as kin selection. It is a powerful theory within evolutionary biology attributed to W. D. Hamilton, which relates to the altruism that is an outgrowth of genetic relatedness. Altruism was difficult to explain evolutionarily. Inclusive fitness explained altruism among kin by emphasizing that the genetic material residing within the individual, is also distributed among the individual’s kin. If an individual survives, so does 100% of his genetics. But if that individual sacrifices his life to save the lives of three brothers, his body dies, but 150% of his genetic material survives in the persons of his three brothers. As such an altruistic dispositions can evolve.
3. Ontogenetic and phylogenetic are a pair of terms, respectively, denoting the development of the individual organism and the evolutionary development of the species.
4. Adaptive diversification will be treated at greater length as it is contrasted with adaptive radiation and discussed as an explanation of personality variation. Suffice it to say at present, adaptive diversification is when evolution selects for a range of types instead of a species specific optimum.
5. Microevolution is contrasted with macroevolution. In microevolution, evolutionary changes occur within a population causing diversity and differences. Nevertheless, change is not sufficient for speciation, such that all members of the population remain within the same species. Macroevolution occurs when evolutionary changes are sufficiently pronounced to cause speciation. Macroevolutionary changes occur in sexually reproducing species when population members can no longer interbreed, or do not longer attempt to do so.
6. Lineage is the genetic line of a family. It is one’s ancestry.
7. The richness of birth order literature can be gleaned through the 3,950 retrieved results obtained on a google scholar search conducted on 30 December 2016 for the words birth order within the title.
8. Deidentification is the process of differentiation that siblings undergo during development. They actively diverge from one another, becoming contrasts with different traits and abilities.
9. Disidentification is similar to deidentification. In the literature, disidentification seems to be the larger process, of which deidentification is an instance. Deidentification is most often use to denote disidentification that is specific to the sibling relationship. Both involve conscious and unconscious divergence between people. A son who never touches alcohol because his father was an alcoholic, is practicing disidentification; he is becoming the opposite of the example set for him. In contrast, identification is imitative. Staying with the same example, it would entail following in the father’s footsteps and becoming an alcoholic as well. In the literature, disidentification seems to be the larger process, of which de identification is an instance.
10. Related to personality through overlapping traits like conscientiousness, life history evolution explains developmental timing, longevity, and maturation rates among other core biological, social, and cultural variables. For an introduction see Stearns (Citation1992). The evolution of life histories (Vol. 249). Oxford: Oxford University Press.
11. Altricial contrasts with precocial. These are ends of a continuum relevant to the state of independence of a new born or hatched organism. On one side are altricial young that, like the human baby, cannot do much for themselves and would starve for want of parental intervention and attention. On the other side is precocial young that, like the Australian Brushturkey, can move, defend and feed for themselves immediately upon hatching.
12. Encephalized refers to the brain size; but brain size as relative to body size. One can calculate a cephalic index or an encephalization quotient that provides a ratio between brain and body.
13. Phylum, order, and family are classifications within the Linnaean taxonomy, which separates all life into the following classifications: domain, kingdom, phylum, class, order, family, genus, and species.
14. The hox gene is a portion of the genome regulating body plan, such as segmenting. It is deeply conserved over evolutionary time. This means that the same genetic regions regulate body plans in humans as in tigers and flies. The number of hox genes varies by organism, as does the number of segments and general body plan, but across a great range of life, it is the hox gene region that regulates body plan.
15. Sympatric refers to having a shared locale. Sometimes speciation or divergence, as described above, results from geographic isolation. Two populations passively drift from one another because they are not interbreeding or otherwise are actively goaded by different ecologically imparted evolutionary pressures to become increasing different. By including the word sympatric, it is to denote that this process of divergence through adaptive diversification is taking place within a single locale. These population members are not diverging and diversifying because they are drifting or subject to different selection pressures from the overarching environment. To the extent that different environments (environmental heterogeneity) is responsible for adaptive diversification, it is not due to large-scale changes in climate, but due to certain proportions of the population being driven into various micro-niches within the social environment.
16. This refers to the ancestral lineage, meaning familial descent from parents to offspring or a group with common descent. This should not be confused with the survival of the species at large.
17. Phenotypic plasticity refers to the ability to alter one’s phenotype without changes to the underlying genotype. Certain fish species undergo transformations of sex referred to as sequential hermaphroditism. For instance, the female sex becomes the male sex. The sex change occurs through hormonal changes interacting with environmental cues. This is an extreme example of morphic or bodily phenotypic plasticity. On the other hand, there is behaviorally based phenotypic plasticity. Related to human personality, one personality and set of genetics is phenotypically plastic to the extent to which it allows a wide range of behavior. People of course can make facultative changes that override temperamental inclinations; they can act differently in different situations. Someone with an anxious temperament can make a conscious effort to take risks, for instance. Nevertheless, personality by definition is a brake on phenotypic plasticity.
18. A reaction norm helps explain the interaction between personality traits and situational demands. It is an elegant solution to the classic state-trait debate that took place between social psychologists and personality psychologists. The social psychologists emphasized the power of the situation to drive behavior irrespective of individual differences. The personality psychologists emphasized fixed trait dispositions that would drive consistent behavior irrespective of situational differences. The reaction norm reconciles these views by describing traits as imparting, not behavior, but thresholds for behavior. Situations are then important in their relation to the fixed threshold. Consider a passive person and an aggressive person. Both may lash out and physically attack someone. However, the passive person may need to be provoked in an extreme way; perhaps by being hit or spit upon. The aggressive person, in contrast may resort to similar physical violence from being bumped into or insulted.
19. As reviewed by Hopper, Rosenheim, Prout, and Oppenheim (Citation2003) bet-hedging can occur within and between generations, can be phenotypically or genotypically expressed, and can be of the conservative or diversified variety. Suffice to state, it is intergenerational, genotypic, diversified bet-hedging presently applied to an understanding of sibling personality variation.
20. Developmental diapause consists of a stage of biological cessation; a type of suspended animation. A kangaroo oocyte can stop growing by avoiding implantation while its older sibling is still suckling, sometimes waiting hundreds of days before accessing maternal resources. Alternatively, there are many seeds and ferns and other plants that can stop growing or reproducing while conditions are unfavorable.
21. Parental investment denotes the degree of direct and indirect resources that are expended on rearing a juvenile offspring to a state of maturity.
22. Dispersal relates to movement away from the parent plant or animal. Dispersal is advantageous to the extent that it limits competition between parent and offspring. Relevant to the present discussion, it can also limit competition between siblings. If one sibling disperses, by for instance joining the armed forces, it leaves the other free to exploit local resources, including family resources.
23. Brood size or clutch size is a measure of how many offspring are produced. Brood size is sometimes used in connection with lifetime reproductive fitness and sometimes in connection with offspring produced in a given breeding season or birthing.
24. Searching exclusively within these 160 citations, only 19 retrievals were made for the word sibling. Even among these, sibling, did not appear in any title. Additionally, reading through the titles of these and the other 141 citing articles, it was not apparent that the explanation of personality variation as bet-hedging was, in any instance, the topic of pursuit.
25. Immune loci relate to regions of the genome with immense variation. Immune profiles vary by blood type or human leukocyte antigen variation or major histocompatibility complex. Immune loci, and immunological differences generally, are relevant to the extent that they are the best example of actively maintained diversity. They also illustrate the necessity for bet-hedging. An immunologically heterogeneous brood subject to a wave of illness is more likely to have surviving members than an immunologically homogenous brood.
26. Disassortative contrasts with assortative. Disassortative denotes active mismatch or heterogeneous coupling; whereas assortative denotes active matching or homogenous coupling. When applied to mating, the former will maximize brood diversity and the later will minimize it.
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Notes on contributors
Steven C. Hertler
Steven C. Hertler is a licensed examining psychologist with a research program centering on personality disorders generally, and obsessive personality specifically, which uses behavioral genetics, evolutionary biology, and behavioral ecology to alternatively explain classic character types. While much of his research has concentrated on personality, additional writings have explored life history evolution, comparative psychology, cross-cultural psychology, and climate as it affects evolved behavior and human nature.