Outline by: Heather Steffani

    Lindsey Stevenson

 Fatima Coley


Pinker Chapter 19


I. The Nature-Nurture Debate – Do genes (nature) or the environment - particularly the influence of parents in childhood (nurture) - mold a human being?


A. What accounts for the variation in a population?

1.      Strict biology

2.      Environmental forces

3.      Combination

      B. The Social Constructionists – nurture has the greatest impact on human  

    development (blank slate)

      C. Behavior geneticists, however, developed a different theory


II. Three Laws of Behavior Genetics


A. The First Law: All human behavioral traits are heritable

B. The Second Law: The effect of being raised in the same family is smaller than  

     the effect of the genes

C. The Third Law: A substantial portion of the variation in complex human

     behavioral traits is not accounted for by the effects of genes or families


III. First Law


A.     Heritable = proportion of variance in a trait that correlates with genetic


1.      About half of the variance in intelligence, personality, and life outcomes is heritable – a correlate or indirect product of the genes

2.   Implies that half of the variation among people is inherited

3.   General intelligence, as well as the five major variations in personality 

      (openness to experience, conscientiousness, extroversion-introversion,   

       antagonism-agreeableness, and neuroticism) are heritable

                  4.   Behavioral traits that reflect the underlying talents and temperaments  

                             of a child are heritable

a. Studies that draw conclusions about the effects of parenting are worthless

i. The correlations might be due to genetics, NOT to parenting styles


B.  Twin studies as supporting evidence for the First Law:

1.      The Mallifert brothers: separated at birth but grew up to have  

strikingly similar lives – more than chance alone can account for

2.      The homes (environments) of identical twins who were separated at 

      birth are no more similar than the homes of fraternal twins separated at 

      birth, yet the identical twins are far more similar


IV. Second Law - The effect of being raised in the same family is smaller than  

     the effect of the genes


A. Shared environment: affects siblings equally

1. Effects are small - makes little or no difference in who we turn out to be as adults

a. Adult siblings are equally similar whether they grew up together  

    or apart

b. Adoptive siblings are no more similar than two random people 

    off the street

c. Identical twins are no more similar than one would expect from 

    the effects of their shared genes

B. Nonshared or Unique environment: individual experience


V. Third Law – A substantial portion of the variation in complex human behavioral    traits is not accounted for by the effects of genes or families, but the unique environment


A. Interactions between the parent and child may account for the unique environment


                        1. Not any one parenting philosophy will work

a.       The individualizing parent tailors the interaction to each child’s   


b.      However, we cannot tell if the child’s outcome is due to the 

      parenting or    the temperament of the child

c.       Even if the child elicits different parenting styles, this would 

      still be an effect of heredity


                        2. However, no perfect interaction to support this

a.       If parent-child interaction explained the unique environment, 

      then the same parenting strategy would have different effects 

      on different children

b.      If the same strategy resulted in the same effects, the variation   

      would be due to the shared environment

c.       Parenting differences within a family are due to the genetic 

      inborn differences of the child


                        3. Birth Order may affect the unique environment

a.       The older child has unidivided attention until the next child is  


                                    b.   Younger children must always compete for attention

                                    c.    Sulloway

i. Older children are more assertive and conscientious while the youngest child is more open to new ideas

ii. This would result in the same personalities both inside and outside of the home

d.      Studies show that neutral observers report different  

      personalities outside of the home


B. Group Socialization Theory: Harris


            1. People model their peers, not their parents

                        a. Obeying parents is not the same as emulating them

                        b. Children struggle for status among peers

                        c. Linguists show that children learn language from their peers


            2. Nature Assumption critics

a. Many people are against this theory because they are social constructionists

b. They don’t pay attention to the fact that heredity explains variance

c. Most critics of this want to blame social ills on dysfunctional families

d. They believe that this view supports social engineering and parental nihilism


C. Pinker’s Theory


1.      Peers are not responsible for personality, but socialization in a peer group may be responsible for the unique environment

a.       If children are sorted into peer groups by inborn traits, then the unique environment would be genetic influence

b.      If children choose their peer group by neighborhood, then the shared environment would be responsible

c.       There may be an interaction between genes and peers, in that children with a predisposition towards violence may become activated in a violent group

d.      Twins with the same genes, parents, and peers still have 50% variation

e.       Harris thinks it may the individual’s role within the peer group


2.      Unique environments are unknown

a.       Chance events and experience may shape personality (accidents, missed flights, etc.)

b.      Animal studies show that twin mice and worm clones show differences

c.       Worms have only 959 cells: we have more potential to be affected by chance


3.      What if the unique environment affects neurodevelopment?

a.       The effects would not necessarily be genetic, and not necessarily environmental, but would be biological

b.      In this case, personality would not be due to free will but to fate

c.       Involuntary chance occurrences would makes us who we are


4.      There is no exact procedure for raising a happy successful child

a.       Each child has there own uniqueness to bring

b.      This does not absolve parents of responsibility for raising the child

c.       It is simply not the parents fault if the child is not perfect

d.      We should spend time with our children because we want to, not to increase neural activity

e.       Children are individuals and partners in relationships, not just products of parent’s  behavior




































Fatima Coley


Heather Stafani


Lindsey Stevenson


Pinker, S. R. (2002). The Blank Slate. New York: Viking.




            The Three laws of behavioral genetics may be the most important discoveries in the history of psychology (Pinker, 2002, p. 372). However, many psychologists have yet to come to grips with the three laws. The main reason that many psychologists have not accepted these laws is the fact that they run contrary to the Blank Slate theory. The Three Laws are as follows:

            -First Law: All human behavior traits are heritable.

            -Second Law: The effect of being raised in the same family is smaller than the         

              effect of the genes.

            -Third Law: A substantial portion of the variation in complex human behavioral                      

              traits is not accounted for by the effects of genes or families (Pinker, 2002,

              p. 373).

The laws are about what make humans who we are and the forces that impinge on humans during childhood. It is thought that childhood is the stage of life when intellect and personality are formed.


The first law states that all human behavioral traits are heritable. Behavioral traits are defined as stable properties that can be measured by standardized psychological tests. For example, intelligence tests may ask people to recite groups of digits forward and backward as well as define certain words that are listed in the vocabulary section of the test. Personality tests ask for the test taker to agree or disagree with statements that describe certain situations. These tests are both valid and reliable, generally yielding the same results each time a person takes the test. 


Once the measurements are made, the variance is calculated. The variance is a number that captures the degree to which the members of a group differ from one another. The degree to which one factor overlaps with another factor can be measured as a correlation coefficient, a number between – 1 and 1, that captures the degree to which people who are high on one measurement are also high on another measurement (Pinker, 2002, p. 374).


Chapter nineteen also discusses heritability as the portion of variance in a trait that correlates with genetic differences. The bigger the difference between the two correlations, the higher the heritability estimate. (Pinker, 2002, p. 374). Identical twins reared apart are highly similar, identical twins reared together are more similar than fraternal twins, and biological siblings are far more similar than adoptive siblings, with heritable values between .25 and .75. However, it is important to remember that concrete behavioral traits that depend on content provided by the home or culture are not heritable. Examples of these behavioral traits are the language you speak, which religion you worship, and your political afflation. Behavioral traits that reflect temperaments and talents are heritable. There are also five ways in which personality can vary. The acronym OCEAN stands for the five personalities: Openness to experience, Conscientiousness, Extroversion, Agreeableness, and Neuroticism.

            There are three built in limitations to behavioral genetic methods. The first is that studies of twins, siblings, and adoptees can help explain what makes people different, but  cannot explain what people have in common or universal human nature. The second limitation is that behavioral genetic methods address the variation within the group of people being examined, not variation between groups of people. The third limitation of behavioral genetic methods is that they only show how traits correlate with genes and not that they are directly caused by the genes. For example, having a certain physical appearance that is dictated by genes can lead to other consequences that are not directly controlled by the genes. We know that tall men, on average, are promoted in their jobs more rapidly than short men, and attractive people, on average, are more assertive than unattractive people.


            Radical scientists have tried very hard to discredit the first law, but have been unsuccessful at doing so. Some scientists interpret the first law as genetic determinism, saying genes are important and environment is insignificant. When it comes to genes, however, people are unable to distinguish between fifty percent and one hundred percent. The worst fallout from the Blank Slate is that people misunderstand the effects of the environment.               


            The Second Law states that the effects of being raised in the same family are smaller than the effects of the genes (Pinker, 2002, p. 378). We are shaped by both our genes and our family upbringing. How our parents treated us while growing up and the home environment we experienced impact our development as individuals. There are two different ways in which the environment affects us. The first is the shared environment, which is what impinges on the individual and siblings alike (Pinker, 2002, p. 378). This environment includes parents, home life, and neighborhoods. The non-shared environment is everything else that may impinge on one sibling and not the other, including parental favoritism, the presence of other siblings, and other unique experiences such as being hit by a car or being on honor roll. The effects of the shared and non-shared environments have been measured using twin studies.  It was found that the effects of the shared environment are small and make up less than 10 percent of the variance.                                                              


            The Third Law states that a substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families (Pinker, 2002, p. 380). In summary, it is the unique experiences of the non-shared environment that make people different and help shape who they are. Genes account for 50% of the variation, shared environment 0%, and non-shared 50%. A finding which supports this viewpoint is that identical twins are 50% similar whether they grow up together or apart.  In 1998, Judith Rich Harris, an unaffiliated scholar, published The Nurture Assumption and addressed the topic of whether or not parents matter. This created tension in the field of evolutionary psychology and sparked a heated debate about how children develop. She stated the three laws and tried to get people to recognize what they are implying. She also stated that the conventional wisdom among experts and laypeople about childrearing is wrong (Pinker, 2002, p. 381).


            The Human Drama crated by Rousseau made children and parents the main characters. Children are the noble savages and one of two things can happen - their upbringing and education can help them blossom or it can saddle them down with the corruption of the world. Twentieth century versions of the noble savage and Blank Slate have kept parents and children at center stage. Many behaviorists have stated that children are shaped by contingencies of reinforcement, and have advised parents not to respond to their children’s distress because it would only reward them for crying and increase their crying behavior (Pinker, 2002, p. 381). On the other hand, Freudians believe that we are shaped by our degree of success in weaning, toilet training, and identification with the parent of the same sex. Freudians also advise parents not to take their infants into bed with them to sleep, because it will cause damaging sexual desires.


            However, everyone has tried to place the blame on mothers. Autism was attributed to the coldness of the mother, schizophrenia to the double binds created by the mother, anorexia on the pressure mothers place on their daughters to be perfect, and low self-esteem to toxic or demanding mothers. Every other problem and social ill has also been attributed to the dysfunctional family. Patients in many psychotherapy sessions spend most of their time going through their childhood to find the roots of their tragedies and triumphs. Parents desire great things for their children they want them to go to school, get good grades, refrain from using drugs, stay out of trouble, and get married. However, genes play the largest role in human development, and even parents with the best intentions can have children who struggle and do not succeed.


            Furthermore, children who grow up together in the same home will not necessarily be similar. Similarities can be accounted for by shared genes, as differences among parents and homes have no predictable long-term effects on the personalities of children. A survey in 1997 reported that family connectedness, meaning close bonds and affection, was proactive against adolescent ills such as drug use and unsafe sex (Pinker, 2002, p. 384). It was also noted that people act differently inside and outside the home. Studies show that, all things being equal, children will turn out the same way no matter what their situation in life. Children growing up without a father have higher incidences of teenage pregnancy and higher drop-put rates, but this does not mean that growing up fatherless causes the problems directly.


            Socialization is the exclusive environment shaper of personality. Socialization deals with acquiring the norms and skills necessary to function in society. All of this takes place in the peer groups children have, which absorb parts of the adult culture while developing values and norms of their own. Children strive to be better children rather than trying to be more like adults. It has also been found that parents do not always socialize a child in the best interest of the child. Thus, being socialized by a peer group is another way of saying “living successfully in society.” It is children above all who are alleged to be blank slates, and that can make us forget they are people (Pinker, 2002, p. 399).