1. Good Genes
Three key bits of conceptual background.
- What “a gene for X” means.
- Competition between genes.
- Popularity of genes.
The concept of “good genes.”
Psychological implications.
- Good genes and sexual attraction.
- Research on “the scent of symmetry”.

2. What “a gene for X" means:
(3 different ways of saying same thing)
- the gene is probabilistically associated with X.
- All other things being equal, having one particular variant of the gene (compared to any other variant of the gene) tends to be associated with a somewhat different outcome on characteristic X.
- When measured across a population, there is a nonzero correlation between (a) variation on the specific gene, and (b) measureable outcome on characteristic X.

3. Oversimplified made-up example: Imagine 100 people…
Risk-taking
One variant of DRD4 gene
Other variants of DRD4 gene
Low 8 40
High 12
(Correlation: r = .08)

4. Competition between genes:
Dawkins: “It is differences that matter in the competitive struggle to survive; and it is genetically-controlled differences that matter in evolution.”
[ “differences” = differences in the effects that different genes (or different variants of genes) have on organisms’ reproductive fitness. ]

5. Oversimplified made-up example: Imagine 100 people…
Reproductive Outcome
One variant of a gene
Another variant of the gene
Produce offspring 25 23
Fail to produce offspring 27

6. Another oversimplified made-up example: Imagine 100 people…
Reproductive Outcome
One variant of a gene
Effect: Dumb
Another variant of the gene
Effect: Dumber
Produce offspring 25 23
Fail to produce offspring 27

7. Popularity of genes:
Dawkins: “Evolution is the process by which some genes become more numerous and others less numerous in the gene pool. It is good to get in the habit, whenever we are trying to explain the evolution of some characteristic, such as altruistic behavior, of asking ourselves simply: ‘What effect will this characteristic have on frequencies of genes in the gene pool?’”
Research within evolutionary psychology is informed by answers to this question:
What specific genetically-based psychological tendencies were likely to have made a gene more ‘popular’ under ecological circumstances that characterized ancestral populations?

8. The concept of “good genes”
Dawkins on “good genes”:
“They have an effect on the embryonic development of each successive body in which they find themselves, such that that body is a little bit more likely to live and reproduce than it would have been under the influence of the rival gene or allele.”
The “goodness” a gene is defined jointly by:
- How well it builds a body.
- Eventual reproductive success of that body.

9. Psychological implications of the evolutionary logic underlying the concept of “good genes”:

10. And so (in the domain of mating):
More reproductively fit bodies
Definition:
“Good genes”
And so (in the domain of mating):
Choose mate who has “Good genes”
Produce more reproductively fit offspring
Positive implications for one’s own reproductive fitness
Evolutionary implication:
Evolution of psychological mechanisms that are sensitive to, and respond positively to, evidence that a potential mate has “good genes”
Psychological implication:
Evidence that a potential mate has “good genes”
“Sexually attractive”

11. Three useful questions to ask:
(As a means of generating testable hypotheses)
Who might be especially sensitive to evidence of “good genes”?
When might they be especially sensitive to this evidence?
What phenotypic traits might provide this evidence?
Evidence that a potential mate has “good genes”
“Sexually attractive”

12. Who might be especially sensitive to evidence of “good genes”?
Women
Because (compared to men):
- Greater obligatory parental investment in offspring.
- Greater reproductive costs associated with “unfit” offspring.
- Greater reproductive costs if choose mate with “bad genes.”
Implication:
- Evidence of perceptual sensitivity to “good genes” most likely to be found among women.

13. When might they be especially sensitive to this evidence?
When the probability of conceiving a child is greatest.
Because:
- It’s when a child is conceived that reproductive costs accrue from choosing a mate with “bad genes.”
One implication:
- Variation across the menstrual cycle in women’s sensitivity to evidence of “good genes”

14. What phenotypic traits might provide this evidence?

15. Reality: Perception: More reproductively fit bodies Symmetry
“Good genes”
Body odor
Symmetry
Perception:
“Sexually attractive”
Body odor

16. Two hypotheses:
Women judge the smell of highly symmetrical men to be more sexually attractive than the smell of less symmetrical men.
This effect varies across the menstrual cycle, and occurs primarily when there is greater probability of conceiving a child.

17. The scent of symmetry (Research by Gangestad & Thornhill, and others)

18. (1) Women generally prefer the smell of symmetrical men.
Two key findings:
(1) Women generally prefer the smell of symmetrical men.
(2) Especially when there is greater probability of conceiving a child.
(Results compiled across three studies; Gangestad et al., 2005)