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Life history theory

Author: Dr Simon Moss

Overview

Some people feel a strong motivation to develop their skills, knowledge, and other capacities or resources. Other people, in contrast, feel a strong motivation to produce offspring while young or engage in many sexual encounters. Life history theory was partly developed to explain this difference across individuals.

According to life history theory (Roff, 1992& Stearns, 1992), individuals must divide their energy and resources across different activities. That is, energy and resources devoted to one task cannot simultaneously be devoted to another task (e.g., Kaplan & Gangestad, 2005& Roff, 2002). Individuals utilize diverse strategies to divide these resources, and these choices underpin many personality traits.

One of the most fundamental choices that individuals must reach is whether to direct these resources to somatic efforts or reproductive efforts. Somatic efforts include activities that are intended to maintain and to extend their body and mind, such as the accrual of knowledge and skills. Reproductive efforts, in contrast, entail courtship, gestation, birth, childcare, and competition to secure mates. Life history strategy reflects which of these two strategies individuals tend to adopt.

Both somatic efforts and reproductive efforts confer key benefits. Somatic efforts, such as learning, enable individuals to generate better offspring in the future. Without these somatic efforts, reproductive efforts might not generate offspring that will thrive. Without reproductive efforts, however, individuals might not generate any offspring. A bias towards somatic efforts is called a slow life history strategy, whereas a bias towards reproductive efforts is called a fast life history strategy: that is, when individuals demonstrate this bias, they often reproduce quickly, soon after the reach sexual maturity.

According to proponents of life history theory, when the environment is hazardous and unpredictable, individuals will tend to devote their resources to reproductive efforts. In these instances, somatic efforts, intended to accrue resources, are futile if death might be imminent. Thus, individuals often seem more irresponsible in these settings (for a related theory or explanation, see socioemotional selectivity).

Examples of life history strategies in animals

Some mammals adopt a fast life history strategy. Tenrecs, for example, related to the hedgehog, reproduce only a few weeks after birth. These animals, therefore, invest their resources and energy into reproductive efforts instead of somatic efforts. In contrast, other mammals adopt a slow life history, such as elephants. Relative to other species, such as chimpanzees, humans also adopt a slow life history, reproducing later in life (Kaplan, Hill, Lancaster, & Hurtado, 2000).

Consequences and correlates of life history strategies

Differential K theory

Differential K theory (Rushton, 1985, 1990, 2004), a variant of life history theory, attempts to characterize all the traits that are associated with fast rather than slow reproductive strategies. Specifically, according to this variant, a strategy called r represents individuals with faster life history strategies, who produce many offspring but cannot devote as many resources to their children. In contrast, the strategy K represents individuals with slower life history strategies, who produce fewer offspring but devote more resources to their children. K is associated with delayed sexual reproduction but also larger brains, lower infant mortality, parental care, and altruism. These traits enhance the willingness or capacity of individuals to devote resources to offspring.

Studies in this field have indeed confirmed that diverse variables, all related to a K inclination, are indeed correlated with each other. Bogaert and Rushton (1989) showed that measures of fast life history strategies, such as sexual permissiveness, are positively associated with family size but negatively associated with altruism.

Similarly, to validate the concept of r and K reproductive strategies, Templer (2008) subjected a variety of variables, such as birth rate, AIDs infection, life expectancy, mean IQ, skin color, and GDP across 129 nations to a factor analysis. One factor explained 73% of the variance. That is, K was associated with slow reproduction and thus a limited birth rate as well as a low incidence of AIDS. Furthermore, K corresponded to investment in resources to enhance the success of offspring, such as high life expectancy and IQ.

Figueredo, Vasquez, Brumbach, and Schneider (2007) uncovered three related factors, all associated with life history theory: K, covitality, and personality. Specifically, a series of scales, associated with life history theory were subjected to a factor analysis, including quality of relationships with parents, children, spouses, and friends, altruism, financial status, advice seeking, persistence, planning, wellbeing, general health, and personality, as gauged by the five factor model. Three factors emerged. The first factor included the traditional K variables, such as quality of relationships, altruism, financial status, advice seeking, persistence, and planning. The second factor, called covitality, was associated with subjective wellbeing and general health. The final factor, derived from the personality measures, entailed conscientiousness, agreeableness, low neuroticism, and high extraversion?-traits that are often regarded as ideal.

These three factors were positively related to each other. Conceivably, individuals who assume a K reproductive strategy strive to develop the traits that attract optimal mates, such as conscientiousness and agreeableness. These results persisted even after socioeconomic factors were controlled, challenging the primacy of social privilege theory--the proposition that advantaged families might afford children with more opportunities to facilitate growth.

Destructive or criminal behavior

Jonason, Koenig, and Tost (2010) examined whether or not life history strategies are associated with the personality traits sometimes called the dark triad: psychopathy (see measures of psychopathy), narcissism, and Machiavellianism. They found that various measures of life history strategy were correlated with psychopathy but not narcissism or Machiavellianism. These relationships were replicated even when indirect measures of life history strategy, such as preference for modest amounts of money now instead of significant money later, were administered.

Likewise, Rowe and Flannery (1994) showed that fast life history strategies, as represented by measures of sexuality, are associated with impulsive, rebellious, and deceitful behavior. Presumably, individuals who strive to accelerate reproduction are not as likely to invest in behaviors that extend relationships and resources (for similar findings, see Rowe, Vazsonyi, & Figueredo, 1997).

Parental behavior

Many studies have demonstrates that fast life history strategies or r traits reduce parental care. That is, individuals who adopt these strategies strive to rear many children rather than devote effort and resources to enhancing the success of each offspring. Consistent with this perspective, Rowe and Flannery (1994) showed that parents who adopted fast life history strategies, such as sexual permissiveness, were not as likely to demonstrate affection towards their children or encourage achievement.

Consumption of food with many calories

Sometimes, people are exposed to scenes or events that indicate that resources may be scarce. They may receive news of an impending economic crisis or observe photographs of deprived people. Consistent with life history theory, when people are exposed to cues that imply scarcity of resources, they become more inclined to eat foods with many calories. Specifically, according to life history theory, when the environment is unstable and hazardous, people strive to secure immediate resources, called a fast strategy, rather than accumulate resources gradually. Therefore, in response to adverse circumstances, they might eat excessively.

These possibilities were posited and validated by Laran and Salerno (2013). In one study, participants were told to sample a novel variant of M&Ms. They were informed these M&Ms are either higher or lower in calories that most M&Ms. In one condition, words that related to harshness, such as survival, persistence, struggle, adversity, were embedded in the background. In the other condition, the words were not related to harshness.

When words that relate to harshness were embedded in the background, participants were more inclined to consume the M&Ms that were touted as high in calories than M&Ms touted as low in calories. Presumably, they wanted to secure as many resources, in this instance calories, as possible. Consistent with this premise, when participants were awarded more resources, such as $1 for participating, this effect dissipated& this money, presumably, diminished the need to secure resources. These findings uncover an astounding implication: If people are more aware that certain foods are high in calories, they may be more likely to consume these items under stress.

Saving or spending money during times of economic hardship

During times of economic hardship, some people become more inclined to save their money whereas other people become more inclined to spend their money. Which of these two strategies people adopt depends on their life history strategies.

Specifically, according to Griskevicius et al. (2013), if people were reared in harsh and unpredictable environments, with scarce resources, they become more inclined to adopt a fast life history under stress. They engage in sex at an earlier stage of life, with more people, rear more children at a younger age, and tend to be more impulsive. These strategies are suitable, because these individuals need to rear many children immediately to withstand the possibility of elevated levels of mortality. In contrast, if people are reared in more abundant environments, they become more inclined to adopt a slow life history. They engage in sex later in life, with fewer people, and delay the rearing of children and other pleasures. These strategies are suitable in stable environments, diminishing the likelihood of inefficient allocation of resources.

These tendencies are especially likely to be activated during times of economic hardship. That is, when resources are scarce, people who adopt a fast life history are especially likely to be impulsive and spend resources, whereas people who adopt a slow life history are especially likely to be cautious and save resources. Griskevicius et al. (2013) conducted three studies that verify this possibility.

For example, in one study, participants were exposed to pictures that epitomize economic recession, such as foreclosure signs, or pictures of nature in the control condition. Next, they completed tasks that gauge impulsive behaviors, such as risk taking and temporal discounting, or the tendency to value immediate gains and losses over future but larger gains and losses. Finally, a measure of socioeconomic status during childhood was included, represented by items like "I felt relatively wealthy compared to the other kids in my school". In wealthier people, exposure to recessions diminished risk taking and temporal discounting. In poorer people, exposure to recession increased risk taking and temporal discounting.

Subsequent studies replicated these findings but with other measures of risk taking, such as tendency to approach luxury goods. Furthermore, one of the studies extended these results but with a physiological measure of exposure to stressful circumstances and thus a fast life history. In particular, the researchers measured a urinary biomarker of oxidative stress: 8OHdG. This marker reflects the extent to which the cellular tissue and DNA of individuals had been damaged. When oxidative stress was high, exposure to recession enhanced risk taking, as measured by the BART. When oxidative stress was low, exposure to recession diminished risk taking.

Antecedents to life history strategies

Hazards in the environment

Several features of the environment affect the life history strategy of individuals. Specifically, when the environment is harsh and unpredictable, a fast life history strategy is adaptive. That is, in these environments, somatic efforts are unsuitable. Individuals might die before they reproduce any offspring at all. Conversely, in safe and predictable environments, a slow life history is adaptive. Somatic efforts increase the likelihood that subsequent reproduction will be successful (e.g., Ellis, Figueredo, Brumbach, & Schlomer, 2009).

Consistent with these premises, species that evolved in harsh or unpredictable environments are more likely to adopt a fast life history strategy. They reproduce as soon as they reach sexual maturity. Furthermore, variations within species can also be explained by these features of the environment (Ellis, Figueredo, Brumbach, & Schlomer, 2009).

As Griskevicius, Delton, Robertson, and Tybur (2011) showed, this theory also applies to humans. Given that fast life history strategies are more adaptive in hazardous environments, humans have evolved to become sensitive to cues that indicate the likelihood of mortality. For example, cues that increase the salience of mortality, such as articles about the escalating murder rate, elicit the desire to produce children earlier, especially if individuals were relatively deprived when young.

Likewise, Low, Hazel, Parker, and Welch (2008) examined the association between life expectancy and average age at which parents tend to produce offspring across 170 nations. In this study, life expectancy represented a gauge of mortality. As hypothesized, in nations with a limited life expectancy, representing elevated levels of mortality, parents generated offspring at a younger age (for related findings, see Dunkel, Mathes, & Decker, 2010). Similar findings have been uncovered across regions of Chicago (Wilson & Daly, 1997).

Perceived hazards

Some individuals tend to inflate the perceived volatility or threat in the environment. For example, individuals who exhibit both an anxious and avoidant attachment tend to assume they will be rejected rather than supported. They feel, therefore, they might not be protected from risks. Consistent with life history theory, women who exhibit these insecure forms of attachment are more likely to seek male sexual partners who desire immediate sexual encounters (e.g., Kruger & Fisher, 2008).

Genes

Several variants of particular genes have been shown to coincide with a fast life history strategy, in which individuals seek immediate gains. In particular, Minkov and Bond (2014) showed that a fast life history strategy is related to variants on three genes: the androgen receptor or AR gene, the dopamine receptor D4 or DRD4 gene, and the 5-HTTLPR region of the SLC6A4 gene. More specifically, these findings show that a fast life history could perhaps be ascribed to genes that increase sensitivity to testosterone, increase sensitivity to dopamine, at least in some regions, and reduced sensitivity to serotonin.

To explore this topic, Minkov and Bond (2014) constructed a database of the genetic constitution and life history strategies of many nations. The genetic constitution was derived from a range of other studies. Life history strategies were partly measured from an index called hypometropia, reflecting murder rates, HIV rates, adolescent fertility, and other facets of risk taking. A measure of temporal orientation that included the tendency to save money and to engage in other responsible behaviors was also included in this measure of life history strategies.

As Minkov and Bond (2014) showed, nations that coincide with fast life history strategies, and more risk taking, tended to coincide with a specific genetic profile. This profile included few CAG repeats on the AR gene, corresponding to high sensitivity to testosterone, the 7 repeat allele on the DRD4, corresponding to high sensitivity to dopamine, and the long allele on the 5-HTTLPR, often associated with less depression.

Controversies

Direction of causality

The direction of causality is often uncertain. That is, some researchers assume that fast life history or K reproductive strategies might evoke impulsive behavior and thus increase crime. However, Griskevicius, Delton, Robertson, and Tybur (2011) showed the opposite direction of causality is possible: When violent crime is rampant, the environment is perceived as unpredictable, eliciting fast life history or K reproductive strategies.

To illustrate, as Griskevicius, Delton, Robertson, and Tybur (2011) demonstrated, fast life history strategies are associated with increased rates of violent but not property crime. Hence, these strategies are not associated with all impulsive acts. Instead, violent crimes may amplify the possibility of mortality, perhaps fostering the need to reproduce as rapidly as possible.

Furthermore, in their second study, reminders of mortality also evoked fast life history strategies. That is, in this study, some participants read about the escalating murder rate, underscoring the risk of mortality. Next, all participants answered questions about their attitudes towards early reproduction: A sample item is "If you were to have a child in the next few years, how would you feel". Finally, participants completed questions that gauge their socioeconomic status, and thus access to resources, during childhood.

Again, if mortality was emphasized, individuals tended to express more positive attitudes towards immediate reproduction. Interestingly, this association was especially pronounced in participants who had been deprived during childhood. Presumably, during childhood, these individuals had developed an inclination towards fast reproductive strategies, presumably because resources were scarce and hence attempts to accumulate these provisions were unlikely to be successful. A subsequent study was similar except exposure to morality also reduced the age at which participants reported they would like to reproduce.

Possible mechanisms

Not many studies have examined the mechanisms that underpin the association between perceptions of the environment and reproductive strategies. That is, not many researchers have examined, for example, how the perception the environment is risky and hazardous might translate into the inclination to reproduce offspring earlier.

Time perspective

Conceivably, time perspective might mediate the association between stable environments and slow life history strategies. That is, when the environment is hazardous, individuals need to focus on their immediate needs?-the need to reproduce. Consequently, they are more inclined to reproduce now rather than later. In contrast, when the environment is stable, individuals can direct their attention to future needs. They are more willing to accrue resources now to support their offspring in the future. Consistent with this possibility, an emphasis on present instead of future needs did mediate the association between hazards in the social environment and interpersonal aggression in one study (Kruger, Reischl, & Zimmerman, 2008).

Cortisol

In addition to time perspective, levels of cortisol, a hormone that coincides with a sense of threat, also seems to mediate the association between perceived hazards in the environment and risk attitudes. In one study, conducted by Schechter and Francis (2010), participants completed measure of risk and uncertainty in the family environment, temporal perspective, attitudes towards risk, and investment in education. Furthermore, salivary levels of cortisol were measured. Risk and uncertainty in the family environment were associated with elevated levels of cortisol, and these levels also related to risky attitudes.

Neural underpinning

Figueredo et al. (2006) attempted to characterize the neural regions that might underpin fast and slow reproductive strategies, called peramorphic and paedomorphic respectively. For example, K strategies demand a need to control immediate impulses, apply rules, and pursue future goals. These needs are fulfilled by the prefrontal cortex (see also Guided activation theory of the prefrontal cortex), a region in which myelination is not complete until early adulthood. Similarly, some of these needs are underpinned by the hippocampus-?a region that enables individuals to adapt their behaviors to accommodate the context and thus to exhibit flexibility and awareness. Therefore, according to Figueredo et al. (2006), these regions might develop more rapidly in people who utilize K strategies. That is, early in life, these individuals develop the capacity to control impulses, pursue goals, and extract resources, such as knowledge, from the environment. These strategies become entrenched and accessible during adolescence. When these individuals develop sexual interests, impulse control, future goals, and sensitivity to environment are already established. Their sexual behavior, therefore, will be more controlled.

In contrast, the nuclear accumbens, ventral tegmental area, and amygdala might develop more rapidly in people who utilize r strategies instead. The nuclear accumbens and ventral tegmental area are very sensitive to potential rewards. The amygdala amplifies arousal. If these regions are developed early, adolescents experience powerful urges to engage in sexual pursuits. These strategies become entrenched early in life.

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