Genetic Influences – Module 5

Intro Psych

Feb 22-24, 2010

Classes #14-15

Module 5: Genetic influences

What influences one’s behavior:

Nature or Nurture?

The age-old debate:

Is it genes or is it the environment?

Principle of Genetics

Master plan for your body….

46 chromosomes

23 chromosomes carried in the egg are paired with 23 chromosomes brought to it by the sperm

Each is composed of a molecule called


DNA is made up of thousands of segments which are called genes

Principle of Genetics

Your sex is determined by the 23 rd pair – the sex chromosomes…

From your mother always an “x”

From your father, you have a 50/50 chance of receiving an x  making you a female

From your father, you have a 50/50 chance of receiving an xy  making you a male

XX = female

XY = male

Gender Differences…

The differences typically found between the sexes are small compared to the similarities…

But when it comes to casual sex…

See next slides…

“I have been noticing you around campus. I find you very attractive.”

Clark & Hatfield (1989)

In this study, students were approached by another student of the opposite sex, who uttered the above statement…

This was followed by one of three invitations:

“Would you go out tonight?” or

“Would you come over to my apartment?” or

“Would you go to bed with me?”







Go Out

Men were even more likely to say “yes” to the sexual invitation

About half of both sexes said

“yes” to the date

Go to Apt.

Go to Bed

Not a single woman said “yes” to the sexual invitation

Variations in Perceptions and Reactions

Compared to women, men perceive more sexuality in an interaction between a man and a woman

This is true whether they are participants or observers

However, men see interactions involving their sister as platonic

Baumeister, Catanese, and Vohs (2001)

Which gender has the stronger sex drive?

Which desires more frequent sex?

Which thinks more about sex?

Which masturbates more often?

Which initiates more sex?

Which gender makes more sacrifices for sex?

The Burger King Study

Townsend & Levy (1990)

Who would you prefer: a well-dressed unattractive person or a good-looking person in a Burger King outfit???

Burger King study:

First, males were pre-rated into 2 groups:

Handsome versus homely

Each were put into 1 of 3 costumes:

Armani suit with Rolex (high status), white t-shirt

(medium status), or Burger King uniform (low status)

They then did the same for females


Identical twins



Not identical

(fraternal) twins

Resemble one another as much as any brother or sister would

Twin Studies

Nature or Nurture?


Do identical twins who are adopted by separate families act like one another more than say two adopted children into the same family who have no genetic influences?

Lets look at some case study research to help us determine this…

“The Jim Twins”

Bouchard (1979)

Thomas Bouchard 

University of Minnesota twin studies – extraordinary similarities between Jim

Springer and Jim Lewis

Uncanny coincidences?

Any limitations to this study?

The Jim Twins

Tested at the Univ of Minnesota



Heart Rate

Brain waves

All virtually alike as the same person tested twice

Voice intonations and inflections were so similar that they could not tell each other’s

Twins Oskar Stohr & Jack Yufe

Bouchard (1979)

Part of Bouchard’s twins study…

Oskar Stohl and Jack Yufe were raised in environments with more obvious differences

Did these guys share a flushing the toilet gene?

Oskar raised by Grandmother in Germany as a Catholic and a Nazi

Jack raised by Father in the Caribbean as a Jew

Share traits and habits galore

Love spicy foods

Sweet liquors

Fall asleep in front of the TV

Flush the toilet before using it

Store rubber bands on their wrists

Dip buttered toast in their coffee

Stohr is domineering toward women and yells at his wife as did Yufe before he and his wife separated

And you thought it was because you were incompatible…

Is there a genetic risk of divorce?

McGue and Lykken (1992)

1516 pairs of same-sex twins

722 MZ and 794 DZ


McGue & Lykken (1992)

Divorce Rate Study

Study divorce rates among 1,500 same sex, middle-age twin pairs

Fraternal twin who is divorced

Odds of other twin divorcing go up 1.6 times (as compared to the odds of a not-divorced twin)

Identical twin who is divorced

Odds of divorcing go up 5.5 times

Estimation that people’s differing divorce risks are about 50% attributable to GENETIC


Adoption Studies

Adopted children are more similar to biological parents than adoptive parents





9,17,Sexuality Why do women’s and men’s sexuality differ?