1. Evolutionary Psychology
The study of evolutionary explanations for shared human behaviors.

2. Topics in Evolutionary Psychology
Why do humans tend to pair off monogamously?
Why do babies tend to develop stranger anxiety at about the same age (8-13 mo.)?
Why are people more likely to develop a phobia of spiders than of guns?

3. Evolutionary psychologists use natural selection to explain behaviors.
What is natural selection?
(1) Genetic differences between individuals in a species causes (2) some organisms to be better suited to their environment. (3)These organisms are more likely to survive and pass on their genes so (4)these traits become more common in successive generations.

4. Why does stranger anxiety appear between 8-13 mo?
Genetic mutations create variation in when stranger anxiety appears:
0 months
3 months
6 months
9 months
1 year
2 years
3 years
Never
(For simplicity, assume stranger anxiety persists for 6-8 months from onset.)

5. Why does stranger anxiety appear between 8-13 mo?
In what way are the infants that develop stranger anxiety at 9-12 months better suited to their environment?
0-6 months – what is the danger in getting stranger anxiety too early?
2 years-never – what is the danger in getting stranger anxiety too late?

6. Gender Differences in Reproduction
Women
Sure the baby is hers
Invests lots of time/energy into each child
Can only make one baby per 9 month period (usually)
Men
Less sure the baby is his
May invest very little time and energy into each child
Have virtually unlimited reproductive potential
Therefore, it is in the woman’s best interest to be choosy about who she mates with, picking only men who are 1) likely to have good genes, and 2) willing and able to provide for her and the child.

7. How does a women know? Good looks often indicates good genes.
- Masculinized physical features correlates positively with better immune function.
- Facial symmetry indicates lack of parasite infection.

8. How does a women know?
- Feminized facial features correlate positively a greater willingness to help raise children.
So do we look for masculine or feminine features?
It depends.