Kevin MacDonald, Ph.D.

CSULB, Department of Psychology

Psychology 661

BEHAVIOR GENETICS

1.) BEHAVIOR GENETICS STUDIES IDIOGRAPHIC DEVELOPMENT
2.) BASIC CONCEPTS:
PHENOTYPE = OBSERVABLE OR MEASURABLE CHARACTERISTICS (E. G., HAIR COLOR, IQ)

GENOTYPE = GENETIC COMPLEMENT OF PERSON (E. G., HAVING A RECESSIVE GENE FOR COLOR BLINDNESS)

POLYGENY: MANY GENES INFLUENCE A TRAIT BUT NO ONE GENE HAS A MAJOR EFFECT (E. G., HUNDREDS OF GENES INFLUENCE IQ AND PERSONALITY)

BEHAVIOR GENETIC CONCEPTS

GENOTYPE -- ENVIRONMENT EFFECTS

ACTIVE GENOTYPE -- ENVIRONMENT EFFECT (NICHE-PICKING): CHILD
SEEKS OUT ENVIRONMENTS AS A RESULT OF GENETIC INFLUENCES (E. G., CHILD WITH A SENSATION SEEKING TEMPERAMENT)

EVOCATIVE GENOTYPE--ENVIRONMENT EFFECT: CHILD EVOKES ENVIRONMENTS AS A RESULT OF GENETIC INFLUENCES (E. G., CHILD WITH
A DIFFICULT TEMPERAMENT EVOKES NEGATIVE RESPONSES IN CAREGIVERS; CHILD WITH A SUNNY DISPOSITION GETS POSITIVE RESPONSES.) Another example: Children with violent natural parents and children with non-violent natural parents are adopted into separate families, and the adoptive families are compared. Children with violent natural parents have adoptive parents who use harsh discipline. Chidlren with non-violent natural parents have adoptive parents who use mild discipline. This "environmental" effect is the result of an evocative genotype--environment effect: Aggressive, difficult children evoke harsh parenting.

PASSIVE GENOTYPE -- ENVIRONMENT EFFECT: CHILD IS PASSIVE RECIPIENT OF ENVIRONMENTS WHICH FIT WITH HIS/HER GENOTYPE. (E. G., INTELLIGENT PARENTS HAVE CHILD WITH GENETIC POTENTIAL FOR
INTELLIGENCE; PARENTS ALSO PROVIDE A GREAT DEALOF
INTELLECTUAL STIMULATION WHICH MESHES WITH THE CHILD'S GENETIC POTENTIAL)

SHARED ENVIRONMENTAL INFLUENCES: ENVIRONMENTAL INFLUENCES SHARED BY CHILDREN IN THE SAME FAMILY.
EXAMPLE: CHILDREN IN SAME FAMILY GO TO SAME SCHOOL, HAVE SAME ALCOHOLIC MOTHER, ETC

UNSHARED ENVIRONMENTAL INFLUENCES: DIFFERENT CHILDREN RECEIVE DIFFERENT ENVIRONMENTS; OR DIFFERENT CHILDREN RESPOND TO THE SAME ENVIRONMENT DIFFERENTLY;
EXAMPLE: DIFFERENT BIRTH ORDER, SEX DIFFERENCES

Reaction Range: The range of phenotypic expression depending on different environments of different quality. Different people have differerent reaction ranges; i.e., they respond differently to the same environments.

IQ score

_____________________________________________
Abusive Poor Average Enriched

Quality of Environment

TWO MODELS THINKING ABOUT POLYGENIC TRAITS:

1.) ADDITIVE MODEL:
TRAIT INFLUENCED BY MANY GENES (POLYGENIC), BUT
EACH GENE ADDS ONLY A LITTLE BIT TO THE PHENOTYPE
GENES HAVE THEIR EFFECT NO MATTER WHAT OTHER
GENES ARE PRESENT

EXAMPLE: SUPPOSE IQ IS INFLUENCED BY 200 GENES,
EACH GENE ADDS EITHER +1 TO YOUR IQ SCORE OR
ADDS 0 TO YOUR IQ SCORE;
THE MORE +1 GENES YOU HAVE, THE HIGHER YOUR IQ.
NOTICE THAT IT DOESN'T MATTER WHAT OTHER GENES ARE
PRESENT; EACH GENE HAS ITS EFFECT INDEPENDENT OF THE GENES AROUND IT.


IQ

__________________________________________________
000 100 200

2. NON-ADDITIVE MODEL (EMERGENESIS) TRAIT INFLUENCED BY MANY GENES (POLYGENIC), BUT THE EFFECT OF EACH GENE DEPENDS ON WHAT OTHER GENES ARE PRESENT.
PARENTS CAN'T PASS SUCH TRAITS TO THEIR CHILDREN EXAMPLES: FACIAL ATTRACTIVENESS

EXAMPLES: CARL FRIEDRICH GAUSS; SEATTLE SLEW; PERSONALITY TRAITS

HERITABILITY

START WITH PHENOTYPIC VARIATION SUCH AS A DISTRIBUTION
OF SCORES ON AN IQ TEST:

SOME OF THIS PHENOTYPIC VARIATION IS DUE TO THE GENETIC
VARIATION (PEOPLE HAVE DIFFERENT GENES);
THIS IS Vg

SOME OF THIS PHENOTYPIC VARIATION IS DUE TO ENVIRONMENTAL
VARIATION (PEOPLE EXPERIENCE DIFFERENT ENVIRONMENTS). THIS IS Ve

THE PHENOTYPIC VARIATION (Vp) THEN EQUALS THE SUM OF THE
GENETIC VARIATION (Vg) PLUS THE ENVIRONMENTAL VARIATION (Ve)

Vp= Vg + Ve

HERITABILITY IS THE PROPORTION OF VARIATION IN A TRAIT
THAT IS CAUSED BY GENETIC VARIATION

HERITABILITY DEPENDS ON SAMPLE BEING STUDIED:

1601: 400 YEARS AGO, ONLY RICH PEOPLE ATE WELL.
RESULT: MOST OF THE PHENOTYPIC VARIATION IN HEIGHT WAS
THE RESULT OF ENVIRONMENTAL VARIATION

H1601 = Vg/(Vg + Ve) = 10/(10 + 90) = 0.100

1995: ALMOST EVERYONE EATS ENOUGH TO ACHIEVE THEIR MAXIMUM HEIGHT
RESULT: MOST OF THE PHENOTYPIC VARIATION IN HEIGHT IS THE RESULT OF GENETIC VARIATION

H2001 = Vg/(Vg + Ve) = 10/(10 + 2) = 0.833


SECOND EXAMPLE: ISLAND X HAS A FAMINE THAT STRIKES ALL;
ISLAND Y IS A PARADISE FOR ALL

QUESTION: IS HERITABILITY FOR HEIGHT HIGH ON BOTH ISLANDS,
LOW ON BOTH ISLANDS, OR HIGH ON ONE AND LOW ON THE OTHER?

ANSWER: HERITABILITY CAN BE HIGH WITHIN EACH ISLAND EVEN THOUGH THE DIFFERENCE BETWEEN THE ISLANDS IS THE RESULT OF
ENVIRONMENTAL DIFFERENCES

THEREFORE: 1.) HERITABILITY CAN CHANGE AS A RESULT OF CHANGES IN THE VARIATION DUE TO THE ENVIRONMENT (i.e., CHANGES IN Ve)


2.) CANNOT GENERALIZE RESULTS TO SAMPLES NOT STUDIED BECAUSE EACH SAMPLE MAY HAVE DIFFERENT GENETIC OR ENVIRONMENTAL SOURCES OF PHENOTYPIC VARIATION
(i.e., DIFFERENT Ve OR DIFFERENT Vg).

AS ENVIRONMENTS BECOME MORE UNIFORM (LOWER Ve),
HERITABILITY IS HIGHER.

SUCCESSFUL SOCIAL PROGRAMS RAISE HERITABILITY
BECAUSE THEY DECREASE VARIATION DUE TO THE ENVIRONMENT.


HERITABILITY INCREASES AS CHILDREN APPROACH ADULTHOOD

1.) CORRELATIONS OF MZ TWINS STAY HIGH WHILE CORRELATIONS FOR DZ TWINS DECLINE

IQ CORRELATIONS FOR MZ AND DZ TWINS
AGE MZ DZ
6 MO .75 .72
12 MO .68 .63
24 MO .81 .73
36 MO .88 .79
4 YR .83 .71
6 YR .86 .59
8 YR .83 .66
15 YR .88 .54

2.) ADOPTION STUDIES
a.) CORRELATION BETWEEN ADOPTIVE PARENTS AND ADOPTED CHILDREN DECLINES FROM .35 TO ZERO

AGE 7: r=0.35 AGE 17: r=0.00

b.) IQ OF ADOPTED CHILDREN DECLINES TO CLOSE TO
THE IQ OF NATURAL PARENTS

AGE 7: IQ OF ADOPTED CHILDREN = 110
AGE 17: IQ OF ADOPTED CHILDREN = 95

EXPLANATION:
ACTIVE GENOTYPE--ENVIRONMENT EFFECT (NICHE-PICKING)
MORE IMPORTANT AS CHILDREN GET OLDER

CONCLUSION: DIFFERENT ENVIRONMENTS WITHIN THE "NORMAL" OR "AVERAGE" RANGE DO NOT HAVE MUCH INFLUENCE ON IQ.

HOWEVER, ABUSIVE ENVIRONMENTS CAN AND DO AFFECT IQ.

AVERAGE VS ABUSIVE ENVIRONMENTS

IQ

________________________________________________________________________
ABUSIVE NORMAL ENRICHED

TYPE OF ENVIRONMENT


ADOPTION STUDY

NP = NATURAL PARENTS;
AP = ADOPTIVE PARENTS
AC = ADOPTIVE CHILDREN

CHILDREN ADOPTED NEAR BIRTH

124
122
120
118
116

AP: X= 120

114
112
110
108
106

AC: X = 110
94
92
90
88
86

NP: X = 90

TESTED AT AGE 7:
REASONS TO SUPPOSE ENVIRONMENTAL VARIATION IS
IMPORTANT:
1.) AVERAGE OF AC (110) > AVERAGE OF NP (90); THIS IS
THE AVERAGE EFFECT OF ADOPTION
2.) POSITIVE CORRELATION BETWEEN AP AND AC

REASONS TO SUPPOSE GENETIC VARIATION IS IMPORTANT:
1.) AVERAGE OF AC (110) < AVERAGE OF AP (120)
2.) POSITIVE CORRELATION BETWEEN AC AND NP