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Free Will: Inheritance and Psychosocial StressA central question in applied psychoneuroimmunology is: How much freedom do we really have to make life-altering choices? Studies of identical twins have often been used to examine inherited vs. environmental contributions to individual behavior and diseases. Monozygotic (from the same egg and sperm) twins are considered to be potentially genetically identical. Dizygotic twins are the product of a different sperm and egg, but born “at the same time” from the same parents. Results of these studies can be thought provoking. Here is a brief look at the results of a few of the many studies. PersonalityGenetic and environmental influences on personality dimensions were assessed in 123 monozygotic and 127 dizygotic pairs of twins (). Broad genetic influences were seen in neuroticism (41%), extraversion (53%), openness (61%), agreeableness (41%), and conscientiousness (44%). Non-shared environmental influences were responsible for most of the environmental influences. Anxiety and DepressionHeritability of anxiety sensitivity was examined on 179 monozygotic and 158 dizygotic twin pairs (). Additive genetic and environmental influences seemed to be the best-fit model, with broad genetic components of 45%. Genetic and environmental influences on heart rate, blood pressure, and self-reported anxiety responses to 4 different types of stress was evaluated in 200 twin pairs (ages 12-44 years, mean = 20) (). Heritability estimates for resting heart rate, diastolic and systolic blood pressure were 65%, 63%, and 58% respectively. Genetic and idiosyncratic environmental factors explained cardiovascular reactivity to the Visual Verbal Test for Conceptual Thought, mental arithmetic, and the cold pressor test. Isometric handgrip stress reactions seemed to be more closely related to family environment. A very recent study (2006) concluded that perceived stress is in part heritable (). They evaluated 103 monozygotic and 77 dizygotic twin pairs Results indicated that summation of both genetic and environmental factors were at work. Depending upon the scale, between 5% and 45% of perceived stress appeared to be hereditable. Genetic and environmental contributions to major depression (MD) in males and females were studied in a volunteer sample of 2,662 Australian Twins (). The mean age for women was 44 years (range = 28 to 89 years; S.D. =12.35), and for men the mean age was 42 years (range = 28 to 84 years; S.D. = 11.23). Three different criteria sets were used to define depression: DSM-III-R major depressive disorder (the broadest definition), DSM-IV major depressive disorder, and severe DSM-IV major depressive disorder (the narrowest definition). Lifetime prevalence using the DSM-III-R, DSM-IV, and severe DSM-IV for women and men were respectively: 31% and 24%; 22% and 16%; 9% and 3%. For both sexes, the relative contribution of environment and genetics were stable for each definition. For women, it seems that genetic factors and unique environmental factors contribute to development of MD: genetic heritability estimates range from 36% to 44%. For men, environmental factors appear to play a much larger role: heritability estimates range was 1% for the narrowest definition to 24% for the broadest, but because of such a small prevalence of depression by the narrow definition, this range may be illusory. Familial aggregation of MD was moderately seen in women and was mostly explained by genetics. In men, familial aggregation of MD was only modest. As defined by the broader definition of the DSM-III, MD was more heritable for women. For both sexes, the environment made a large contribution to MD. Depression symptoms seem to be largely inheritable, and the relative influences vary by genetics and age (). Four hundred eleven twin pairs between the ages of 8 and 16 years were evaluated. Over the whole age range, the best data fit was for an additive genetic model. But, for the ages range of 8 to 11 the symptoms were explainable by shared environmental factors, while the ages 12 to 16 were highly associated with genetic factors. DiseaseLife style, stress, and genetic influences on peptic ulcer disease were investigated in 13,888 same sex twin pairs (4,307 monozygotes and 9,581 dizygotes) (). Thirty-nine percent of the liability to peptic ulcer disease was explainable by genetics, and 61% was explainable by individual environmental influences. These environmental factors included smoking and stress in men and analgesic use in women. Self-reported health status is a significant predictor of morbidity, mortality, and use of health services. Heritability of self-reported health was explored in 4,638 male twin pairs (). The best-fit statistical model estimated that 39.6% of the variance in health could be ascribed to genetics. In a model that included health condition (e.g., diabetes, cancer, heart disease), genetics accounted for 32.5% of the variance while health condition contributed 15.0%. Body mass index (BMI) appears to be a product of inheritance and non-shared environment (). A study of 1,233 Danish twin pairs found that heritability in males was approximately 46% for ages 46-59 and 61% for ages 60-76. Females showed genetic influences at 77% for 46-59 and 75% for 60-76 years of age. So, it would seem that females manifest more genetic influences than males for BMI, and genetic influences increase with age in males but not females. PsychopathologyRelationships between parenting and later psychopathology was investigated for the relative contributions of genetic and environmental factors in 546 monozygotic and 390 dizygotic female twin pairs and their parents and the offspring of some of these twins (). The researchers concluded that genetic factors in parents and children are more important for providing warmth rather than for providing protection or authority. Providing parenting is influenced both by genetics of parental temperament and attitudes stemming from the parent’s family of origin. Elicitation of parenting seems to be influenced by temperamental traits of the offspring that are also partially under genetic control. Young Danish twins (34,142) were screened for eating disorders by self-reported mailed questionnaires (). Monozygotic and dizygotic twins differed significantly in concordance rates. Genetic influence rates were 48% for narrowly defined anorexia nervosa, 52% for broadly defined anorexia nervosa, and 61% for bulimia nervosa. How these self-reports relate to clinical diagnoses is unknown, Binge eating was explored in 8,045 twins ages 18-31 from a Norwegian twin registry (). No correlations with sex were seen, but the model was low in ability to do so. Heredity apparently correlated with binge eating at 41% and individual environment correlated at 59%. Substance abuseEnvironmental and genetics influences on substance initiation, use and problem use in adolescents were examined by the Colorado Center for the Genetics and Treatment of Antisocial Drug Dependence (). Data was processed on 345 monozygotic and 337 dizygotic twin pairs, 306 biological sibling pairs, and 74 adoptive sibling pairs. Moderate to substantial genetic roles were found for all except alcohol use and any drug use. Problem use appears to be more heritable than initiation and use. Modest to moderate shared environmental influences were seen on substance initiation, use, and problem use. Issues of peer pressure, access to substances, and sibling interaction may be the explanation behind the environmental influences found only in twin pairs for tobacco initiation, alcohol use, and any drug use. Genetics appeared to play a stronger role and environment a weaker role for alcohol and any drug initiation, use, and problem use. For males more than females, genetics had a greater role than environment for tobacco use and problem use, and marijuana initiation. Environmental influences shared only by twin pairs were tobacco use, any drug use, and alcohol use. Lifetime tobacco, alcohol and other substance use was examined in 327 monozygotic and 174 same-sex dizygotic twin pairs born in Minnesota and aged 17 to 18 at the time they were assessed (). They found that the heritability of the likelihood of tobacco, alcohol, and other substance use in males was approximately 59%, 60%, and 33%. For females, the estimates were 11%, 10%, and 11% respectively. These results did not appear to show a statistically different effect by sex. Shared environmental effects were considerable yet not statistically significantly greater in females than males (71%, 68%, and 36% vs. 18%, 23%, and 23% respectively). The researchers believe that there is a common underlying substance use factor that is influenced 23% by genetics, 63% by shared environmental factors, and 14% by non-shared environmental factors. Summary of dataAlthough all the data is not presented here (but it can be found in our reference volume Psychoneuroimmunology for Health Sciences) genetics has been seen to contribute to the variance in at least one twin-based study for each of the following (percent contribution):
So, what the heck does this all mean?We would suggest that the data shows that we have considerable latitude for psychosocial choices. For most of the parameters discussed above, environment plays at least an equal part with genetics in determining our characteristics. Nevertheless, it should be pointed out that this environment may have already limited some of our choices. For example, dysfunctional families may have had pathological influences in our formative years on our self-image, perception of stress, and our coping mechanisms. The choices we make will be further constrained by our mobility, physical limitations, monetary resources, the attitudes and social mores of the society or even the country in which we were born or live. And, sometimes the capriciousness of fate steps in. Given all of these considerations, it behooves us to arm ourselves with the best evidence for making our health-related choices. For the field of psychoneuroimmunology it is our aim to provide you with the tools to help you make those choices. |
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