Helping and Reproductive Value of the Recipient
KeywordsReproductive Success Future Reproduction Male Relative Current Reproduction Paternal Uncertainty
Considerable evidence support the basic assumption of Hamilton’s inclusive fitness theory showing that individuals behave altruistically towards kin (see Burnstein 2005. for a review). However, there are several factors, which affect the probability of behaving altruistically toward kin, like reproductive value of the recipient. The “evolutionary logic” behind this bias corresponds with the variability of an individual’s potential to reproduce offspring. The reproductive value of an individual is determined through its current and future reproduction, i.e., its expected summarized contribution to the next generation of the population. Future reproduction (or residual reproductive value) represents the investment of the individual of its own growth and survivorship. It is suggested that higher investment in current reproduction reduces future reproduction, because it expropriates resources from growth and survivorship. However, resources invested in growth will benefit the individual in the future through the increase of number of offspring and reproductive episodes (Fisher 1930).
Several disorders affect the reproductive success of the individual through limiting mating frequency or fertility which induce a decrease of the likelihood of passing on genes. Accordingly, individuals are less likely to act altruistic toward kin with such reproductive limitations because the lower probability of passing on the gene responsible for altruism. There is some evidence to support this notion, for example, it was observed that mothers provided less maternal care (e.g., length of breast feeding) to weakly infants (i.e., low birth weight) compared to healthy infants (Bereczkei 2001).
However, reproductive limitations differ in their conspicuousness and in the extent to which they affect the reproductive value of the individual. For example, a homosexual young adult male dispose no visible reproductive limitation, although his homosexuality could (but not necessarily do) impede the production of offspring. The aim of this entry is to provide a short review of factors affecting the reproductive value of an individual.
Sex of the Recipient
In several studies, female relatives are favored over male relatives by altruists of both sexes suggesting the significant influence of the sex of the recipient in evaluation of the cost-benefits ratio of an altruistic act (e.g., Burnstein et al. 1994). Although women have limited reproductive capacity, because the internal fertilization, they can be absolutely certain about being genetically related to their child. Contrarily, men always face the (hypothetical) possibility that the child has been fathered by another man. This phenomenon is called paternal uncertainty. Accordingly, it is more beneficial to help a female relative compared to a male relative, since the helper can be more certain that the recipient transmits their shared genes.
Age of the Recipient
Aging affects the reproductive success of the individual and accordingly it also influences the extent to which the recipient can utilize the benefits of the help. As a general rule, older individuals are supposed to be less efficient at converting resources into offspring. In good agreement with this hypothesis, Burnstein et al. (1994) found that the preference of female kin over male kin in helping fades away with the increase of the age of the target person. At the age level close to the onset of menopause, or over this period, the likelihood of reproducing offspring drops significantly, thus it is no more favorable to support female over male kin. Moreover, supporting younger individuals results a more lasting impact, because it exerts its effect over a longer period of the life span. However, very young and very old individuals are more vulnerable to damages of reproductive value, like diseases or neglect, and therefore when benefits of altruism are significant, very old and very young individuals are less likely to receive help. On the other hand, if the age of puberty is close, or already began, the likelihood to receive help increases. Those of intermediate age are the most likely to receive help from relatives, because their contribution to current reproduction is the highest. Therefore, in life or death situations, the probability of kin altruism is curvilinear with the age of the recipient, but in everyday situations (when benefits of altruism are nearly 0) this correspondence reverses and aid flows to the youngest and oldest individuals (Burnstein et al. 1994). Although, helping relatives with limited reproductive value has moderate direct benefits, but it advertise our generosity, which can enhance the likelihood to get help from others (Zahavi and Zahavi 1997).
Status and Resources of the Recipient
In the case of differences in access to resources, helping is less costly for high-status individuals, thus the cost-benefit ratio of altruism is differing according to the resources available for the individual. Therefore, individuals can use their resources to manipulate others, thus wealth can lead to receiving more help but only under certain circumstances: in life or death situations, altruists do not discriminate between close kin according their status or resources, but they do in relation to distant kin. For example, high-status siblings receive help as likely as low-status siblings, while wealthy cousins get help more often than less wealthy ones (Burnstein et al. 1994).
Physical Status of the Recipient
Numerous physical features can affect reproductive success through influencing attractiveness, for example, male height, which is associated with reproductive success (Nettle 2002). Another visible and significant factor is weight: both men and women prefer mates who are of average weight over thin or obese mates. Weight affects the reproductive functions of both men and women; therefore, it has a significant impact on the reproductive success of men and women. Women, who are excessively thin or overweight bear difficulties with their fertility. Underweight men display a decreased sperm count, whereas overweight is associated with lower levels of testosterone in men (see Fitzgerald and Colarelli 2009). In addition, there are several more, persistent physical health indicators (e.g., deformity or dysmorphism) which also influence kin altruism (see introduction of this section). Altruist evaluate the apparent health status of the recipient, and in life or death situations they tend to favor healthy kin over those poor in health, but in everyday situations this effect is reversed and altruists prefer relatives in poor health over healthy relatives (Burnstein et al. 1994).
Mental Health of the Recipient
More or less apparent reproductive limitations are mental disorders or problems in everyday mental functioning. Psychological problems can impair reproductive success, for example, depression (and many other psychological disorders) is associated with decreased sexual motivation or limited social functioning. These characteristics are less appealing for mates, thus helping kin with mental disorder produces a lower fitness payoff than helping kin who are not mentally ill. Moreover, individuals with mental illness are presumably less altruistic or cooperative, thus helping these people is likely to remain unrequited (see Fitzgerald and Colarelli 2009).
Sexuality and Reproductive Value
If an individual engage in sexual activities without reproductive possibilities, it is a sexual reproductive limitation. For example, as mentioned above, homosexuality (and to a lesser extent, bisexuality) is a behavior which is associated with a decreased probability to reproduce offspring. Sexual orientation is less conspicuous and not necessarily rigorous reproductive limitation since homosexuals are physiologically capable of reproducing (see Fitzgerald and Colarelli 2009).
In accordance with Hamilton's inclusive fitness theory, assessing cost-benefit ratio of helping involves proximate mechanisms to evaluate the reproductive value of the recipient. Recipient’s handicap in reproduction can lead to a less advantageous return to fitness of the helper, hence altruists have to be able to take these limitations into account. Accordingly, individuals are less likely to help those, with a limited reproductive potential. However, there are many different forms of reproductive limitations, which may have different impact on reproductive value of the individual. In good agreement with this notion, empirical evidence showed that people are more willing to risk their lives to save a relative with a temporary reproductive limitation than a relative with more persistent limitation (Fitzgerald and Colarelli 2009).