Psychology 305 1

Psychology 305A: Theories of Personality

Lecture 6

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1. What are the basic principles of evolutionary theory?

2.How have the basic principles of evolutionary theory been adapted to the study of personality?

3. What hypotheses has the evolutionary approach generated regarding (a) altruism and (b) sex

differences in the desire for sexual variety?

The Biological Perspective

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By the end of today’s class, you should be able to:

1. describe natural selection, sexual selection, intrasexual selection, and intersexual selection.

2. generate examples of distinct forms of selection.

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3. define the term “inclusive fitness.”

4. review research supporting evolutionary hypotheses related to altruism.

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5. define the terms “reproductive capacity” and “minimal parental investment.”

6. review research supporting evolutionary hypotheses related to sex differences in the desire for sexual variety.

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What are the basic principles of evolutionary theory?

• According to Darwin’s theory of evolution, across successive generations of a species, organisms develop characteristics that enhance their ability to survive and reproduce (i.e., adaptations).

• Darwin maintained that there are two processes through which organisms develop these characteristics:

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1. Natural selection

A process through which organisms develop characteristics that enhance their ability to survive.

Organisms that possess characteristics that enhance their ability to survive are more likely to live to reproductive age and to pass these characteristics on to the next generation.

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Through this process, characteristics that enhance the organism’s ability to survive increase in frequency across successive generations of the species until they characterize virtually all members of the species.

Offspring that inherit these characteristics, in turn, are more likely to live to reproductive age and to pass the characteristics on to yet the next generation.

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Example: The preference for sweet, fatty, and salty foods among humans.

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2. Sexual selection

A process through which organisms develop characteristics that enhance their ability to reproduce.

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Darwin maintained that there are 2 mechanisms through which sexual selection occurs:

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(a) Intrasexual selection

Occurs when same sex organisms compete for the sexual interest of the opposite sex.

Organisms that possess characteristics that enable them to “win” the sexual interest of the opposite sex are more likely to mate and to pass these characteristics on to the next generation.

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Offspring that inherit these characteristics, in turn, are more likely to mate and to pass the characteristics on to yet the next generation.

Through this process, the characteristics that enabled the victors to win increase in frequency across successive generations of the species until they characterize virtually all members of that sex of the species.

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Example: The Ariel Hover Display of the male Bronzed Cowbird.

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(b) Intersexual selection

Occurs when organisms of one sex prefer to mate with organisms of the opposite sex that possess specific, desirable characteristics.

Organisms that possess characteristics that are desirable to the opposite sex are more likely to mate and to pass these characteristics on to the next generation.

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Offspring that inherit these characteristics, in turn, are more likely to mate and to pass the characteristics on to yet the next generation.

Through this process, characteristics that are desirable to the opposite sex increase in frequency across

successive generations of the species until they characterize virtually all members of that sex of the species.

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Example: The brilliant plumage of the male peacock.

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How have the basic principles of evolutionary theory been adapted to the study of personality?

• Evolutionary psychologists believe that our basic psychological processes are the product of natural selection and sexual selection.

• Our psychological processes are seen as adaptations to the survival and reproductive challenges faced by our

ancestors.

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What hypotheses has the evolutionary approach generated regarding altruism?

• Altruism: Refers to a desire to help others, even at the risk of one’s own well-being.

• Hamilton (1964)’s theory of inclusive fitness: Organisms are motivated to enhance their own survival and

reproductive potential, as well as the survival and reproductive potential of those who are genetically similar to them.

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• On the basis of Hamilton’s theory, evolutionary psychologists generated two hypotheses regarding altruism.

Hypothesis B: Helping behaviour increases as the reproductive potential of one’s kin member (i.e., relative) increases.

Hypothesis A: Helping behaviour increases as thedegree of genetic overlap increases between the helper and the recipient.

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• Burnstein et al. (1994) tested both hypotheses in the U.S. and Japan.

Asked participants to imagine that people were asleep in a rapidly burning building.

Told participants they could only rescue one person.

Asked participants to indicate who they would be most willing to save.

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In order to test Hypothesis A, participants were presented with people who varied in their degree of

genetic overlap with the participant.

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Tendency to Help

Degree of Genetic Overlap

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Highly Related Not Related

Results consistent with Hypothesis A.

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In order to test Hypothesis B, the researchers conducted 2 studies. In the first study, participants were presented with kin members who varied in age.

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In one condition, participants were asked to respond under the assumption that they lived in an environment with plentiful resources:

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Tendency to Help

Age of Kin (Years)

1 10 18 45 75

Results consistent with Hypothesis B.

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In another condition, participants were asked to respond under the assumption that they lived in an environment with extremely limited resources:

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Tendency to Help

Age of Kin (Years)1 10 18 45 75

Results consistent with Hypothesis B.

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In the second study designed to test Hypothesis B, participants were presented with kin members who varied in health.

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Tendency to Help

Degree of Genetic Overlap

.50 .25 .125

Highly Related Not Highly Related

Results consistent with Hypothesis B.

Healthy

Sick

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What hypotheses has the evolutionary approach generated regarding sex differences in the desire for sexual variety?

• Females and males differ in their reproductive capacity and the minimal parental investment that is required to produce an offspring.

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• Reproductive capacity: Males > Females

Males: Can bear an almost unlimited number of offspring across the lifespan (maximum recorded to date: 900).

Females: Can bear 8 – 12 offspring across the lifespan.

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• Minimal parental investment: Females > Males

Females: Carry and nourish fetus for 9 months; nurse child after birth; during lactation, ovulation is suppressed.

Males: Engage in copulation.

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• Given sex differences in reproductive capacity and minimal parental investment, evolutionary psychologists maintain that males and females have developed different

strategies to maximize their reproductive potential.

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• Males: Employ strategies that enable them to copulate with as many females as possible.

• Females: Employ strategies that restrict copulation to males who are able to provide care and resources for the offspring.

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• Following from these strategies, evolutionary psychologists have generated 2 interrelated hypotheses regarding sex differences in the desire for sexual variety:

Hypothesis A: Males are less discriminating in their selection of sexual partners than females.

Hypothesis B: Males are inclined to seek more sexual partners than females.

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• Studies that have tested these hypotheses include:

Clark and Hatfield (1984)

Had confederates approach students of the opposite sex on campus.

After saying “Hi, I’ve been noticing you around campus lately and I find you very attractive,” the confederates asked the students 1 of 3 questions:

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“Would you go out on a date with me tonight?”

“Would you go back to my apartment with me tonight?”

“Would you have sex with me tonight?”

Percent Who Agreed

Male Female

50 55

69 6

75 0

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These results are consistent with Hypothesis A.

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1. What are the basic principles of evolutionary theory?

2.How have the basic principles of evolutionary theory been adapted to the study of personality?

3. What hypotheses has the evolutionary approach generated regarding (a) altruism and (b) sex

differences in the desire for sexual variety?

The Biological Perspective

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