Kevin MacDonald, Ph.D.

An Evolutionary Perspective on Human Fertility

EVOLUTIONARY TRENDS IN PARENTING:

LOW INVESTMENT (PRIMITIVE MAMMALIAN PATTERN)
- MANY YOUNG
- LARGE LITTERS
- SHORT LIFESPAN
- SMALL BRAINS COMPARED TO BODY SIZE
- SHORT PERIOD OF DEPENDENCE ON PARENTS
- MALES NOT INVOLVED IN PARENTING
- MOTHER AND OFFSPRING ARE FAMILY UNIT
HIGH INVESTMENT
- FEW YOUNG
- LONG LIFESPAN
- LARGE BRAINS
- LONG PERIOD OF DEPENDENCE ON PARENTS
- HIGH LEVEL OF PLASTICITY AND LEARNING ABILITY
- MALE INVESTMENT IN OFFSPRING
- EVOLUTION OF PAIR-BONDING MECHANISMS

Ecologists term selection for high investment K-selection. K-selected species have high-quality offspring able to compete successfully in a highly competitive environment. Humans are the quintessential K-selected species.

Conceptualizing Reproductive Strategies:

Theory I: K-Selection and Adversity Selection

Adversity Selection: Environmental adversity requires high levels of parental investment; e.g., the Ice Age. Humans are an adversity-selected species. They are also a K-selected species.

Predictions:
Adversity-Selected and K-selected species (including humans) will respond to environmental scarcity with delayed maturation, delayed reproduction, and lowered fertility.

Adversity-Selected and K-selected species (including humans) will respond to environmental richness and abundance with earlier maturation, earlier reproduction, and higher fertility.

Hypotheses #1: Stressors of all kinds uniformly result in a lowered tempo of maturation, including age of menarche.

Data: Effects of malnutritionon puberty; ballet dancers

Hypothesis #2: ECONOMIC PROSPERITY leads to INCREASED FERTILITY
DEPRESSION OF 1890'S AND 1930'S -- LOWERED AGE OF MARRIAGE, LOWERED FERTILITY

PROSPERITY OF 1950'S led to INCREASED FERTILITY FERTILITY

Historical data from Europe: Population booms and busts associated with economic booms and busts. Virginia Abernethy has many similar examples from around the world.

Theory II: Evolved Motive Dispositions and Fertility

I. EVOLUTIONISTS HAVE PROPOSED THAT SOCIAL STATUS SEEKING IS AN EVOLVED MOTIVATIONAL SYSTEM:

a.) SOCIAL STATUS IS A UNIVERSAL FEATURE OF MATE CHOICE

b.) SOCIAL STATUS IS HIGHLY CORRELATED WITH REPRODUCTIVE SUCCESS IN TRADITIONAL SOCIETIES

c.) THE POWER OF THE SOCIAL STATUS SEEKING MOTIVE: BRITISH WORKING CLASS ENGAGING IN 'ECONOMICALLY IRRATIONAL BEHAVIOR' TO PRESERVE TRAPPINGS OF SOCIAL STATUS

II. ANOTHER EVOLVED MOTIVE DISPOSITION: CHILDREN UNIVERSALLY VIEWED AS DESIRABLE BUT COSTLY ITEMS (PAUL TURKE)

III. CONFLICT BETWEEN MOTIVATIONAL SYSTEMS: SOCIAL STATUS VERSUS HIGH FERTILITY

Theory III: Belsky's theory of reproductive strategies

VARIATION IN REPRODUCTIVE STRATEGIES

LOW INVESTMENT HIGH INVESTMENT

Stressful environment, Unstressful environment, lots of resources,
few resources, precarious economically secure

a. FAMILY CONTEXT

MARITAL DISCORD SPOUSAL HARMONY
SINGLE PARENTING PATERNAL COMMITMENT
NEGLECTFUL PARENTING
SIBLING REARING

b. CHILDREARING IN INFANCY/EARLY CHILDHOOD

HARSH, REJECTING,                        WARM, RESPONSIVE
INSENSITIVE
UNSTIMULATING                              STIMULATING
NO PARENT-CHILD PLAY                 PARENT-CHILD PLAY

c. PSYCHOLOGICAL/BEHAVIORAL DEVELOPMENT

INSECURE ATTACHMENT                 SECURE ATTACHMENT
MISTRUSTFUL INTERNAL                 Trustful Internal Working Model
WORKING MODEL
OPPORTUNISTIC                                    RECIPROCALLY REWARDING
INTERPERSONAL STYLE                     INTERPERSONAL STYLE

d. SOMATIC DEVELOPMENT

EARLY LATER
MATURATION/PUBERTY MATURATION/PUBERTY

e. REPRODUCTIVE STRATEGY

EARLIER SEXUAL ACTIVITY                     LATER SEXUAL ACTIVITY
UNSTABLE PAIR BONDS                             STABLE PAIR BONDS
LOW INVESTMENT                                      HIGH INVESTMENT
PARENTING                                                           PARENTING

f. INTELLIGENCE

LOWER HIGHER

BELSKY, J., L. STEINBERG, & P. DRAPER (1991) CHILDHOOD EXPERIENCE, INTERPERSONAL DEVELOPMENT, AND REPRODUCTIVE STRATEGY: AN EVOLUTIONARY THEORY OF SOCIALIZATION. CHILD DEVELOPMENT, 62, 647-670.

Supporting data: It has been difficult to unambiguously test the theory. It is true, e.g., that contemporary people in economically difficult situations tend to adopt a low-investment strategy. However, this is difficult to separate from genetic influences, i.e., that people who are genetically inclined to lower-investment parenting have children with the same tendencies. Genetic studies show age of menarche and the other life history traits are heritable. However, Belsky et al. point out that the correlational patterns hold even if controled for biological mother's age of menarche. This is a crude control for genetic influences, but there may be something to it. At a theoretical level it seems odd to suppose that people would reproduce more when things got more difficult. Seems like a bad strategy in the Ice Age. The virtue of the adversity selection/K-selection theory is that such conditions would lead to cutting back reproduction.

Theory IV: Learning mechanisms

1.) An intriguing way in which evolved systems may influence fertility is via interaction with social learning mechanisms. One proposal is that people's fertility decisions are influenced by their perceptions of acceptable living standards and social class status during childhood. From an evolutionary perspective, this suggests a mechanism in which there are learning biases that are activated during a 'sensitive period' of development. This hypothesis fits with data showing that people whose living standards exceed their childhood standards are prone to higher fertility. Those who haven't yet achieved the living standard of childhood delay fertility. This is a learning mechanism, but clearly one that depends on an evolved sensitive period for learning certain information.

2.) Another social learning mechanism was proposed by Surbey (1990) who suggested that the presence of an unrelated male, typically a step-father, during a developmentally sensitive period triggers a low-investment style of reproduction in children. Some animal species show this phenomenon: When an new male comes around, the young females enter puberty early and mate with the new male. The evidence for such a mechanism in humans is inconclusive at this time because of the difficulty of disentangling genetic proclivities to low-investment parenting from a mechanism in which low-investment parenting is induced by an environmental trigger. However, step-father presence is associated with earlier puberty and a low-investment reproductive style in girls (Ellis & Garber, 1999), while positive family interactions are associated with later puberty (Ellis et al., 1999). Again, this is a learning mechanism, but clearly one that depends on an evolved sensitive period for learning certain information.

3.) Evolutionary biases in social learning may also affect fertility decisions in situations where high-status models adopt low fertility behavior. Boyd and Richerson (1985) have developed an evolutionary model of cultural diffusion in which adaptive or maladaptive cultural practices may spread via modeling processes. High-status models are particularly effective in influencing people. An evolutionary hypothesis proposes that social norms of low fertility result from evolved social learning biases toward imitating the behavior of elites or socially conventional behavior. Again, this is a learning mechanism, but clearly one that depends on an evolved tendency for imitating high-status models. The theory is necessarily incomplete because it does not provide a mechanism for the original transformation of the fertility behavior of elites.

Conclusion: Fertility, like aggression, is too important to leave to only one variable or influence. Several of these variables are probably influential. However, there is a conflict between the adversity selection/K-selection perspective and Belsky's reproductive strategy perspective. They can't both be true.