Heritability, Shared and Non Shared Environment

Heritability, Shared and Non Shared Environment

 

According to Turkheimer (2000) there are three nearly unanimously accepted "laws of behavior genetics".

 

First Law: All human behavioral traits are heritable

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

 

The study of the way that people in a family are similar or different has three major parts. The first is how much of their personality depends on their genetics (h). The second is how much of their personality depends on their shared environment (E₂). And the last is how much of their personality is different because of their non-shared environment (E₁).

 

Rowe and Plomin (1981) reviewed the family study research data from various researchers and described the following findings. Dizygotic (DZ) twins have a personality correlation of about .28 due to shared genetics and E₂ (shared environmental factors) while non-twin siblings have a correlation of about .12. This means that about 75% of the variance in personality is due to E₁ (non-shared environmental factors). Intelligence correlations show more effect of E₂ but still leave 40% of the variance to non-shared factors.

 

Table # 1

Correlation of Personality and Cognition (Rowe & Plomin, 1981)

	Sibling type
Trait	DZ Twin	Non-twin sibling	MZ Twin	Adopted Siblings
Personality	.28	.12	.50
Cognition	.62	.34	.86	.25

 

 

The weakness of the family study using DZ twins and non-twin siblings is that it does not separate out the genetic from the shared environmental factors. To separate these factors researchers turn to the adoption study to remove the shared environment factor or to the MZ twin study (monozygotic or identical) to remove the genetic factor.

 

The correlation of personality between DZ twins is due to having about half (h/2) of their genes in common which is referred to as heritability (h) and having a shared environment (E₂). The correlation of DZ twins (CDZ) = (h/2)+ E_2. However there is no way pull out the heritability factor using just this formula. MZ twins have 100% of their genes in common so the correlation of MZ twins (CMZ) = h+ E_2. Algebraically combining, removing the common environment, and solving for heritability results in a formula where heritability is roughly equal to twice the difference between the CMZ and CDZ (Plomin, DeFries, McClearn, & McGuffin, 2001)

CMZ = h + E₂

CDZ = (h/2) + E₂

CMZ – CDZ = h – (h/2) + E₂ – E₂

CMZ – CDZ = (2h/2) – (h/2)

CMZ – CDZ = h/2

h = 2(CMZ-CDZ)

 

Once heritability is found then it can be used in the CDZ formula to find out what part of the CDZ is due to shared environment. The final element, non-shared environment (E₁), represents the part that is different between the siblings will be the parts already known subtracted from 1.

 

Shared genetics (h)        

2(CMZ-CDZ)                     

2(.50-.28)                         

.44                             

 

Shared environment (E₂)

CDZ - ( h /2 )

.28 - (.44 / 2)

.06        

 

Non-shared environment (E₁)

1 – h – E₂

1 - .44 - .06

.50

 

Table # 2

Factors Affecting Personality and Cognition between DZ Twins

	Calculated values
Trait	Shared genetics (h)	Shared environment (E2)	Non-shared environment (E1)
Personality	.44	.06	.50
Cognition	.48	.38	.14

 

As adopted siblings share environment but no genetics the estimated E₂ for DZ twins and the measured E₂ for adopted siblings should be similar (DZ twins = .38 vs Adopted = .25).

 

Rowe and Plomin (1981) suggest that the large effect of non-shared environment (E₁) may imply that E1 events are random such as meeting a person outside the home that inspires a child to do something different, an illness not shared, birth order, peers not shared by siblings, pre-natal trauma, deidentification (child chooses to be different), or differing parental or sibling interactions.

 

Turkheimer (2000) discussed the research of Plomin and Daniels (1987) research where they suggested that there are two kinds of nonshared environment. The first is objective where the event is actually not experienced by both siblings. The second is effective or subjective where the event is experienced by both siblings but interpreted differently. Turkheimer claims that this conjecture is false based on a review of 43 studies. I will leave his disagreement for discussion in a later post so that I can now discuss some of the research supporting that the non-shared environment can be analyzed if not completely then in parts.

 

The research of Daniels, Dunn, Furstenberg, & Plomin (1985) discusses non-shared environmental effects of events that are actually happening in the family unit or with shared peers. This seems to be an example of effective or subjective non-shared environment (E₁). They first reviewed other researcher's data that had indicated that non-twin siblings had about a .20 correlation in personality. Their nonshared environment included such factors as sex, age, and birth order which only accounted for 1% of variation and de-identification between first and second born children and between siblings of the same sex.

 

The researchers proposed that the perceived and real differences in how people within the family and shared peers treated an individual affected the personality of the individual. Research with siblings age 11-17 years-of-age indicates that they perceived that the environmental influence of their differing relationships with family and peers was different than that of their siblings. Parents of the siblings reported minor differences in their relationship with each of their children. They found that perceived differences in Maternal Closeness, Siblings Friendliness, and Peer Friendliness account for 4% to 13% of the variance in sibling adjustment. They also found that there was only about a .15 correlation of emotional adjustment between siblings.

 

The researchers (Daniels, Dunn, Furstenberg & Plomin, 1985) tested nine environmental areas:

 

1. Family Cooperation,
   

1. Family Stress,
   

1. Parental Rule Expectations,
   
2. Parental Chore Expectations,
   
3. Maternal Closeness,
   
4. Paternal Closeness,
   
5. Child's Say in Decisions,
   
6. Siblings Friendliness (parental perception & child perception)
   
7. Peer Friendliness (parental perception & child perception).
   

 

 

[To be continued next weekend]

 

 

 

 

 

Citations

Daniels, D., Dunn, J., Furstenberg, F., & Plomin, R. (1985). Environmental differences within the family and adjustment differences within pairs of adolescent siblings. Child Development, Vol 56(3), Jun 1985. pp. 764-774. Retrieved PsycINFO July 5, 2009. http://libproxy.uta.edu:2066/login.aspx?direct=true&db=psyh&AN=1985-25181-001&site=ehost-live

Plomin, R., DeFries, J.C., McClearn, G.E., & McGuffin, P. (2001). Behavioral Genetics. New York: Worth. P 350.

Rowe, D.C. & Plomin, R. (1981). The importance of nonshared (E-sub-1) environmental influences in behavioral development. Developmental Psychology, Vol 17(5), Sep 1981. pp. 517-531. Retrieved July 5, 2009 PsycARTICLES. http://libproxy.uta.edu:2066/login.aspx?direct=true&db=psyh&AN=1981-32469-001&site=ehost-live

Turkheimer, E. (2000). Three laws of behavioral genetics and what they mean. Current Directions in Psychological Science 9(5), (2000): 160-164. Retrieved July 2, 2009, http://www.psychologicalscience.org/newsresearch/publications/journals/currdir/cd9_5_5.pdf