Everything is heritable; another twin study showing pervasive heritability of political beliefs/attitudes. "Are Political Orientations Genetically Transmitted?", Alford et al 2005; excerpts:
"We test the possibility that political attitudes and behaviors are the result of both environmental and genetic factors. Employing standard methodological approaches in behavioral genetics--specifically, comparisons of the differential correlations of the attitudes of monozygotic twins and dizygotic twins--we analyze data drawn from a large sample of twins in the United States, supplemented with findings from twins in Australia. The results indicate that genetics plays an important role in shaping political attitudes and ideologies but a more modest role in forming party identification; as such, they call for finer distinctions in theorizing about the sources of political attitudes.
But what is the physical process by which a genetic allele could shape a political attitude? If there is any connection at all, is it not that the effect is so small that it can be safely ignored? And even if this is not the case, in light of potentially troubling normative implications such as biological determinism, is it not best to ignore relationships between genes and social behavior? It is difficult for many outside the biological sciences to understand how it is even possible for genes to influence behavior, so a brief discussion is in order. Genes provide instructions for the production of proteins, which are built and identified by a specific combination of amino acids (which in turn are constructed from complex organic molecules). As such, each protein has a chemical sequence that then interacts with other chemicals in the body, sometimes reacting directly with these other chemicals but often serving as enzymes that facilitate but are not themselves altered by chemical reactions. If a gene coding for a particular enzyme is absent, the chemical reaction it is meant to enhance will occur with much less efficiency. For example, a gene for the enzyme tryptophan hydroxylase-2 (Tph2) facilitates production of the neurotransmitter serotonin in the brain, but a certain form of this gene (which varies from the standard form by a single amino acid) produces about 80% less serotonin and people with this mutant allele appear to be significantly more likely to suffer from unipolar depression (Zhang et al. 2005).
Still, the connection is rarely so simple that a given genetic allele can be seen as causing a certain behavior. More typically, findings in modern behavioral genetics reveal the effect of genes to be interactive rather than direct, let alone determinative. To provide one illustration, in humans there is a gene on chromosome 17 involved with serotonin reuptake (5-HTT). As is often the case with genes, 5-HTT has a long allele and a short allele. Mice have a parallel gene, and in that species the short form had previously been connected to listless, depressive behavior. Scientists were eager to determine if such a correlation between the short form of 5-HTT and depression was present in humans. In a long-term study of the health records of nearly 1,000 New Zealanders whose 5-HTT alleles were known, it was found that major episodes of depressive behavior were not much more prevalent among those with the short form. But then the researchers combined genetics and the environment; specifically, they interacted each subject's 5-HTT allele with the number of high-stress events (romantic calamities, bankruptcies, deaths of loved ones, etc.) experienced in that individual's life. They found that those who had a high number of such events and who had the short form of 5-HTT were significantly more likely to display behaviors associated with depression compared to either those experiencing few high stress events or those with the long form who suffered through a comparably large number of high-stress events (see Caspi et al. 2003). In this particular case, genotype did not make people behave a certain way; rather, it influenced the extent to which their behavior was contingent on the environment--and this pattern likely will apply to all sorts of other human activities. Whether the behavior of interest is depression, cooperation, fear response, or susceptibility to drug addiction, some people are more sensitive than others to particular features of their environment, and genetics, far from determining behavior, influences its sensitivity.
This assertion that the effect of genetics is measurably distinct for MZ and DZ twins, while the effect of the environment is either equivalent or at least randomly distributed around equivalence, is crucial to everything that follows from twin research. It is important therefore to raise and consider the criticisms of this fundamental assumption. The arguments come in two essential varieties. The first is that MZ twins, genetics aside, experience a more similar environment because they are treated more similarly than are DZ twins. This would seem particularly telling for childhood socialization, where, for example, parents might show less of a tendency to treat MZ twins as individuals compared to DZ twins. The second is that MZ twins, genetics aside, interact with each other more throughout life than do DZ twins. This would seem to be of particular importance for adult socialization, where closer adult contact between MZ twins might lead us to expect a greater degree of environmentally induced similarity than we would see for the more distant DZ twins. Both caveats have been subject to sustained and varied investigation and neither has been found to hold up under empirical scrutiny. The argument of more similar treatment fails on several fronts. Parents frequently miscategorize their twins (DZ twins are often believed by their parents to be MZ twins) and the differential correlation persists in these instances of miscategorization. In other words, the degree of correspondence between MZ twins surpasses that of DZ twins even in the large subpopulation of twins thought by their parents to be MZ twins (Bouchard and McGue 2003; Bouchard et al. 1990; Plomin 1990). The contention that MZ twins have closer or more frequent contact than DZ twins turns out to be at best irrelevant. The correlation between the frequency of contact between twins and the similarity between twins on all attitudinal and behavioral variables tested, including conservatism, is slight and actually negative (Martin et al. 1986). In other words, twins in greater contact with their cotwins are not more likely to share the same attitudes and behaviors, so even if MZ twins have more contact than DZ twins, this contact is not the cause of any elevated correlations. But the most powerful refutation of both of these criticisms comes in recent studies utilizing MZ and DZ twins raised apart. These studies uniformly validate MZ and DZ differences found in earlier studies of twins raised together. Arguments about the relative degree of shared environmental effects between MZ and DZ twins simply offer no credible explanation if the twins in question have been raised apart (Bouchard 1998; Bouchard et al. 1990)...In addition, as adult MZ twins living apart age, they tend to become more, not less, similar (Bouchard and McGue 2003), a finding that is difficult to reconcile with the belief that only the environment matters.
Heritability is typically estimated by subtracting the correlation for DZ pairs from the correlation for MZ pairs and then doubling the resulting difference. At one extreme, if the correlations are the same for MZ and DZ pairs, suggesting that genetic similarity plays no role in similarity for that particular trait, then the result will be an estimate of heritability of zero. At the other extreme, a purely genetic additive trait should produce a correlation of .5 for DZ pairs and 1.0 for MZ pairs, resulting in an estimate of heritability of 1.0 (1.0 − .5 = .5, and 2 ∗ .5 = 1.0). In a similar way, we can estimate the influence of shared environment, as opposed to shared genetic material, by doubling the correlation for DZ pairs and then subtracting the correlation for MZ pairs. Again, a purely genetic additive trait should produce a correlation of .5 for DZ pairs and 1.0 for MZ pairs, resulting in an estimate of the impact of shared environment of zero (2 ∗ .5 = 1.0, and 1.0 − 1.0 = 0). At the other extreme, if the correlations are the same for MZ and DZ pairs, suggesting that genetic similarity plays no role in similarity for that particular trait, then the result will be an estimate of the impact of shared environment that is equal to the MZ or DZ correlation (e.g., if MZ = DZ = .4, then 2 ∗ .4 = .8, and .8 − .4 = .4). Whatever is left over is taken to be attributable to the unshared environment.
In the classic political science studies of socialization (see, especially, Jennings and Niemi 1968, 1991 and Tedin 1974), the focus has been on the correlation between the attitudes of parents and their children. In terms of the three sources of trait variability outlined above, as informative as it is, this design does not allow for an unambiguous estimation of any of the three categories. The correlation between a parent and a child arises from a combination of shared genes, shared environment, and parental socialization (an indirect form of shared environment in which the parent's attitudes provide a path from the parent's environment to the child's environment), all of which are pressures toward similarity in parent-child attitudes. The failure of this parent-child correspondence to reach +1.0 presumably reflects the pressure toward dissimilarity coming from the unshared environment, but since the genetic similarity of a parent-offspring pair is only .5, there is as much genetic dissimilarity as there is similarity. Thus, trait dissimilarity, like trait similarity, is an undetermined mixture of genetic and environmental influences.
These techniques have been valuable for epidemiological traits, intelligence, personality, social attitudes such as those connected to religion, psychological interests, and behaviors such as risk-taking propensities (for a thorough review, see Bouchard and McGue 2003). Of most interest to us are the findings pertaining to social attitudes and behaviors. At first, researchers were so confident that social attitudes were not heritable that they employed such items as controls. Quickly they discovered that other controls would have to be found because most social attitudes consistently displayed a surprising measure of heritability (see, e.g., Crelia and Tesser 1996, Scarr and Weinberg 1981, and Tesser 1993)....Heritability estimates calculated by previous researchers for attitudes associated with psychological conservatism are quite high, while the relevant models typically show little or no effect for shared environment (the remainder is likely the result of nonshared environmental factors). Notably, these findings come from studies of twins in settings as disparate as Australia, Virginia, and Minnesota, and the findings of the Minnesota study, utilizing twins reared apart, conform well to the other studies of twins raised together (for a summary, see Bouchard and McGue 2003). 5 Careful studies of adopted children confirm the finding that genetics matter more than parentally created environment in influencing social attitudes and behaviors, personality traits, and intelligence. [Adoption studies measure the correlation of biological parents and adopted children where the biological parents have had no contribution to the rearing (environment) of the child. The most recent adoption study, utilizing surveys of Korean-American adoptees randomly assigned to families in the United States, concludes that roughly 75% of variance in children's educational attainment is attributable to the educational attainment of their biological parents, and only 25% is attributable to the adoptive parents, thus dramatically confirming the earlier findings of a substantial correlation between biological parents and adopted children and a surprisingly paltry correlation between adoptive parents and children (Sacerdote 2004). This parallels, with an entirely distinct methodology, the basic finding of the twin studies (see Plomin et al. 1997, 1998 and Rhee and Waldman 2002).]
Based on behavioral geneticists' study of religion, it seems that group identification is something that is heavily influenced by the environment, especially shared environment, and is mostly unconnected to genetics. Children of Methodists are likely to be Methodists not because there is a gene for Methodism or even a personality particularly oriented toward Methodism, but because of parental socialization. Thus, even as attitudes connected to religiosity and religious beliefs and activities (e.g., Sabbath observance, church authority, belief in heaven, religious fundamentalism, frequency of attendance) were found to be shaped more by genetic inheritance than by parental views on those issues (for details, see Bouchard et al. 1999, Eaves, Martin, and Heath 1990, Maes et al. 1999, and Martin et al. 1999), identification with a particular religious group was shaped more by socialization and almost not at all by genetics. We expect to find a similar pattern with political party identification. Children are eager to belong to the groups their parents belong to and parents are frequently eager to encourage children in this regard.
For example, the heritability of conservatism is frequently assessed (see, e.g., Bouchard et al. 1990, Eaves, Eysenck, and Martin, 1989, and Martin et al. 1986), and even though conservatism is viewed by the scholars who do twin studies more as a psychological trait than a political ideology, measures of it include political items. Of most relevance here is the Wilson-Patterson (W-P) Attitude Inventory. This inventory is administered by presenting subjects with a short stimulus phrase such as death penalty or royalty and eliciting a simple agree, disagree, or uncertain response...We were granted access to the data for the W-P items in the United States and were able to conduct comparable, though more limited, twin correlation analyses from published results of an Australian study. 7 The U.S. study included information on thousands of twin pairs in Virginia, supplemented with twin pairs recruited through the cooperation of AARP. A subset of these twins and their relatives has been asked questions regarding their social attitudes, including numerous items from the W-P inventory.
Even the quickest glance at the results in Table 1 is enough to set aside the traditional view that genes do not play any role in explaining political attitudes. All 28 of the MZ correlations are larger than their corresponding DZ correlations, and in every case the difference is statistically significant at the .01 level. Far from typically being at or near zero, none of the 28 heritability estimates falls in the single digit range, and more than half of the 28 items have heritability estimates of .3 or more. Heritability ranges from a high of .41 to a low of .18, all suggesting that the influence of heredity on political attitudes is very real, and given the diverse range of items included here, this genetic influence is also pervasive.
...The results for the overall index are presented in Table 2 and clearly support a powerful role for heredity in influencing conservatism, at least as measured by the W-P inventory. The estimate for heritability is .43, higher than for any of the individual items. The estimate for shared environment is .22, falling within the upper range of the individual items, while the estimate for unshared environment is only .35, falling very near the bottom of the range for individual items. The overall picture is again a very strong role for heredity and a less powerful, but clear role for shared environment.
Party identification is distinct among U.S. political attitudes both in our conception of it as an identification, and hence as something at least potentially distinct from simple item evaluation, and in its established tendency to correlate well between parent and child (see Jennings and Niemi 1968). This distinctiveness is apparent in Table 2. As we expected, the pattern for party identification is nearly the exact reverse of that for the average attitude item. Heritability for party affiliation is relatively low (r = .14), while shared environment is much stronger (r = .41). Note also that not one of the 28 W-P items has an average heritability that is as low as that for party affiliation, and likewise, not one of the 28 items has an average coefficient for the impact of shared environment that is as high as that for party affiliation. Clearly, party identification is, at least for the United States, a different sort of beast than reactions to issue items.
The immediate empirical question is how much of a role assortative mating plays in political issue positions. A quick answer can be found by looking at the interspouse polychoric correlations for the individuals included in the Virginia 30K study. The average interspouse polychoric correlation for the 28 items is .41 and the individual correlations range from a low of .26 for Censorship to a high of .64 for School Prayer. While some of this interspouse similarity could plausibly be attributed to persuasion effects taking place after mate choice rather than to assortative mating, the levels of similarity are probably too high to dismiss assortative mating entirely. This is confirmed by a preliminary look at the impact of controlling for assortative mating on these 28 attitude items. The Virginia 30K study includes data for parents of twins in the study, including parents' individual responses to the same W-P items that the twins responded to...The results for a partial correlation analysis controlling for parental agreement are reported in Table 2, on the row just below the results for the overall index. For MZ twins the issue of whether their parents agree or disagree on a particular item makes little difference (.65 without control versus .64 after partialing out the effect of parental agreement). In contrast, the correlation between DZ twins decreases modestly when the impact of parental agreement is removed (.43 without control versus .37 after partialing out the effect of parental agreement). Further, the tendency of assortative mating to deflate estimates of heritability while inflating estimates of the impact of shared environment is clear. Without controls, the estimate of heritability for the overall index is .43 and the average estimate of the impact of shared environment is .22. When the impact of parental agreement is partialed out, the average estimate of heritability rises to .53, and the average estimate of the impact of shared environment drops to .11."
"We test the possibility that political attitudes and behaviors are the result of both environmental and genetic factors. Employing standard methodological approaches in behavioral genetics--specifically, comparisons of the differential correlations of the attitudes of monozygotic twins and dizygotic twins--we analyze data drawn from a large sample of twins in the United States, supplemented with findings from twins in Australia. The results indicate that genetics plays an important role in shaping political attitudes and ideologies but a more modest role in forming party identification; as such, they call for finer distinctions in theorizing about the sources of political attitudes.
But what is the physical process by which a genetic allele could shape a political attitude? If there is any connection at all, is it not that the effect is so small that it can be safely ignored? And even if this is not the case, in light of potentially troubling normative implications such as biological determinism, is it not best to ignore relationships between genes and social behavior? It is difficult for many outside the biological sciences to understand how it is even possible for genes to influence behavior, so a brief discussion is in order. Genes provide instructions for the production of proteins, which are built and identified by a specific combination of amino acids (which in turn are constructed from complex organic molecules). As such, each protein has a chemical sequence that then interacts with other chemicals in the body, sometimes reacting directly with these other chemicals but often serving as enzymes that facilitate but are not themselves altered by chemical reactions. If a gene coding for a particular enzyme is absent, the chemical reaction it is meant to enhance will occur with much less efficiency. For example, a gene for the enzyme tryptophan hydroxylase-2 (Tph2) facilitates production of the neurotransmitter serotonin in the brain, but a certain form of this gene (which varies from the standard form by a single amino acid) produces about 80% less serotonin and people with this mutant allele appear to be significantly more likely to suffer from unipolar depression (Zhang et al. 2005).
Still, the connection is rarely so simple that a given genetic allele can be seen as causing a certain behavior. More typically, findings in modern behavioral genetics reveal the effect of genes to be interactive rather than direct, let alone determinative. To provide one illustration, in humans there is a gene on chromosome 17 involved with serotonin reuptake (5-HTT). As is often the case with genes, 5-HTT has a long allele and a short allele. Mice have a parallel gene, and in that species the short form had previously been connected to listless, depressive behavior. Scientists were eager to determine if such a correlation between the short form of 5-HTT and depression was present in humans. In a long-term study of the health records of nearly 1,000 New Zealanders whose 5-HTT alleles were known, it was found that major episodes of depressive behavior were not much more prevalent among those with the short form. But then the researchers combined genetics and the environment; specifically, they interacted each subject's 5-HTT allele with the number of high-stress events (romantic calamities, bankruptcies, deaths of loved ones, etc.) experienced in that individual's life. They found that those who had a high number of such events and who had the short form of 5-HTT were significantly more likely to display behaviors associated with depression compared to either those experiencing few high stress events or those with the long form who suffered through a comparably large number of high-stress events (see Caspi et al. 2003). In this particular case, genotype did not make people behave a certain way; rather, it influenced the extent to which their behavior was contingent on the environment--and this pattern likely will apply to all sorts of other human activities. Whether the behavior of interest is depression, cooperation, fear response, or susceptibility to drug addiction, some people are more sensitive than others to particular features of their environment, and genetics, far from determining behavior, influences its sensitivity.
This assertion that the effect of genetics is measurably distinct for MZ and DZ twins, while the effect of the environment is either equivalent or at least randomly distributed around equivalence, is crucial to everything that follows from twin research. It is important therefore to raise and consider the criticisms of this fundamental assumption. The arguments come in two essential varieties. The first is that MZ twins, genetics aside, experience a more similar environment because they are treated more similarly than are DZ twins. This would seem particularly telling for childhood socialization, where, for example, parents might show less of a tendency to treat MZ twins as individuals compared to DZ twins. The second is that MZ twins, genetics aside, interact with each other more throughout life than do DZ twins. This would seem to be of particular importance for adult socialization, where closer adult contact between MZ twins might lead us to expect a greater degree of environmentally induced similarity than we would see for the more distant DZ twins. Both caveats have been subject to sustained and varied investigation and neither has been found to hold up under empirical scrutiny. The argument of more similar treatment fails on several fronts. Parents frequently miscategorize their twins (DZ twins are often believed by their parents to be MZ twins) and the differential correlation persists in these instances of miscategorization. In other words, the degree of correspondence between MZ twins surpasses that of DZ twins even in the large subpopulation of twins thought by their parents to be MZ twins (Bouchard and McGue 2003; Bouchard et al. 1990; Plomin 1990). The contention that MZ twins have closer or more frequent contact than DZ twins turns out to be at best irrelevant. The correlation between the frequency of contact between twins and the similarity between twins on all attitudinal and behavioral variables tested, including conservatism, is slight and actually negative (Martin et al. 1986). In other words, twins in greater contact with their cotwins are not more likely to share the same attitudes and behaviors, so even if MZ twins have more contact than DZ twins, this contact is not the cause of any elevated correlations. But the most powerful refutation of both of these criticisms comes in recent studies utilizing MZ and DZ twins raised apart. These studies uniformly validate MZ and DZ differences found in earlier studies of twins raised together. Arguments about the relative degree of shared environmental effects between MZ and DZ twins simply offer no credible explanation if the twins in question have been raised apart (Bouchard 1998; Bouchard et al. 1990)...In addition, as adult MZ twins living apart age, they tend to become more, not less, similar (Bouchard and McGue 2003), a finding that is difficult to reconcile with the belief that only the environment matters.
Heritability is typically estimated by subtracting the correlation for DZ pairs from the correlation for MZ pairs and then doubling the resulting difference. At one extreme, if the correlations are the same for MZ and DZ pairs, suggesting that genetic similarity plays no role in similarity for that particular trait, then the result will be an estimate of heritability of zero. At the other extreme, a purely genetic additive trait should produce a correlation of .5 for DZ pairs and 1.0 for MZ pairs, resulting in an estimate of heritability of 1.0 (1.0 − .5 = .5, and 2 ∗ .5 = 1.0). In a similar way, we can estimate the influence of shared environment, as opposed to shared genetic material, by doubling the correlation for DZ pairs and then subtracting the correlation for MZ pairs. Again, a purely genetic additive trait should produce a correlation of .5 for DZ pairs and 1.0 for MZ pairs, resulting in an estimate of the impact of shared environment of zero (2 ∗ .5 = 1.0, and 1.0 − 1.0 = 0). At the other extreme, if the correlations are the same for MZ and DZ pairs, suggesting that genetic similarity plays no role in similarity for that particular trait, then the result will be an estimate of the impact of shared environment that is equal to the MZ or DZ correlation (e.g., if MZ = DZ = .4, then 2 ∗ .4 = .8, and .8 − .4 = .4). Whatever is left over is taken to be attributable to the unshared environment.
In the classic political science studies of socialization (see, especially, Jennings and Niemi 1968, 1991 and Tedin 1974), the focus has been on the correlation between the attitudes of parents and their children. In terms of the three sources of trait variability outlined above, as informative as it is, this design does not allow for an unambiguous estimation of any of the three categories. The correlation between a parent and a child arises from a combination of shared genes, shared environment, and parental socialization (an indirect form of shared environment in which the parent's attitudes provide a path from the parent's environment to the child's environment), all of which are pressures toward similarity in parent-child attitudes. The failure of this parent-child correspondence to reach +1.0 presumably reflects the pressure toward dissimilarity coming from the unshared environment, but since the genetic similarity of a parent-offspring pair is only .5, there is as much genetic dissimilarity as there is similarity. Thus, trait dissimilarity, like trait similarity, is an undetermined mixture of genetic and environmental influences.
These techniques have been valuable for epidemiological traits, intelligence, personality, social attitudes such as those connected to religion, psychological interests, and behaviors such as risk-taking propensities (for a thorough review, see Bouchard and McGue 2003). Of most interest to us are the findings pertaining to social attitudes and behaviors. At first, researchers were so confident that social attitudes were not heritable that they employed such items as controls. Quickly they discovered that other controls would have to be found because most social attitudes consistently displayed a surprising measure of heritability (see, e.g., Crelia and Tesser 1996, Scarr and Weinberg 1981, and Tesser 1993)....Heritability estimates calculated by previous researchers for attitudes associated with psychological conservatism are quite high, while the relevant models typically show little or no effect for shared environment (the remainder is likely the result of nonshared environmental factors). Notably, these findings come from studies of twins in settings as disparate as Australia, Virginia, and Minnesota, and the findings of the Minnesota study, utilizing twins reared apart, conform well to the other studies of twins raised together (for a summary, see Bouchard and McGue 2003). 5 Careful studies of adopted children confirm the finding that genetics matter more than parentally created environment in influencing social attitudes and behaviors, personality traits, and intelligence. [Adoption studies measure the correlation of biological parents and adopted children where the biological parents have had no contribution to the rearing (environment) of the child. The most recent adoption study, utilizing surveys of Korean-American adoptees randomly assigned to families in the United States, concludes that roughly 75% of variance in children's educational attainment is attributable to the educational attainment of their biological parents, and only 25% is attributable to the adoptive parents, thus dramatically confirming the earlier findings of a substantial correlation between biological parents and adopted children and a surprisingly paltry correlation between adoptive parents and children (Sacerdote 2004). This parallels, with an entirely distinct methodology, the basic finding of the twin studies (see Plomin et al. 1997, 1998 and Rhee and Waldman 2002).]
Based on behavioral geneticists' study of religion, it seems that group identification is something that is heavily influenced by the environment, especially shared environment, and is mostly unconnected to genetics. Children of Methodists are likely to be Methodists not because there is a gene for Methodism or even a personality particularly oriented toward Methodism, but because of parental socialization. Thus, even as attitudes connected to religiosity and religious beliefs and activities (e.g., Sabbath observance, church authority, belief in heaven, religious fundamentalism, frequency of attendance) were found to be shaped more by genetic inheritance than by parental views on those issues (for details, see Bouchard et al. 1999, Eaves, Martin, and Heath 1990, Maes et al. 1999, and Martin et al. 1999), identification with a particular religious group was shaped more by socialization and almost not at all by genetics. We expect to find a similar pattern with political party identification. Children are eager to belong to the groups their parents belong to and parents are frequently eager to encourage children in this regard.
For example, the heritability of conservatism is frequently assessed (see, e.g., Bouchard et al. 1990, Eaves, Eysenck, and Martin, 1989, and Martin et al. 1986), and even though conservatism is viewed by the scholars who do twin studies more as a psychological trait than a political ideology, measures of it include political items. Of most relevance here is the Wilson-Patterson (W-P) Attitude Inventory. This inventory is administered by presenting subjects with a short stimulus phrase such as death penalty or royalty and eliciting a simple agree, disagree, or uncertain response...We were granted access to the data for the W-P items in the United States and were able to conduct comparable, though more limited, twin correlation analyses from published results of an Australian study. 7 The U.S. study included information on thousands of twin pairs in Virginia, supplemented with twin pairs recruited through the cooperation of AARP. A subset of these twins and their relatives has been asked questions regarding their social attitudes, including numerous items from the W-P inventory.
Even the quickest glance at the results in Table 1 is enough to set aside the traditional view that genes do not play any role in explaining political attitudes. All 28 of the MZ correlations are larger than their corresponding DZ correlations, and in every case the difference is statistically significant at the .01 level. Far from typically being at or near zero, none of the 28 heritability estimates falls in the single digit range, and more than half of the 28 items have heritability estimates of .3 or more. Heritability ranges from a high of .41 to a low of .18, all suggesting that the influence of heredity on political attitudes is very real, and given the diverse range of items included here, this genetic influence is also pervasive.
...The results for the overall index are presented in Table 2 and clearly support a powerful role for heredity in influencing conservatism, at least as measured by the W-P inventory. The estimate for heritability is .43, higher than for any of the individual items. The estimate for shared environment is .22, falling within the upper range of the individual items, while the estimate for unshared environment is only .35, falling very near the bottom of the range for individual items. The overall picture is again a very strong role for heredity and a less powerful, but clear role for shared environment.
Party identification is distinct among U.S. political attitudes both in our conception of it as an identification, and hence as something at least potentially distinct from simple item evaluation, and in its established tendency to correlate well between parent and child (see Jennings and Niemi 1968). This distinctiveness is apparent in Table 2. As we expected, the pattern for party identification is nearly the exact reverse of that for the average attitude item. Heritability for party affiliation is relatively low (r = .14), while shared environment is much stronger (r = .41). Note also that not one of the 28 W-P items has an average heritability that is as low as that for party affiliation, and likewise, not one of the 28 items has an average coefficient for the impact of shared environment that is as high as that for party affiliation. Clearly, party identification is, at least for the United States, a different sort of beast than reactions to issue items.
The immediate empirical question is how much of a role assortative mating plays in political issue positions. A quick answer can be found by looking at the interspouse polychoric correlations for the individuals included in the Virginia 30K study. The average interspouse polychoric correlation for the 28 items is .41 and the individual correlations range from a low of .26 for Censorship to a high of .64 for School Prayer. While some of this interspouse similarity could plausibly be attributed to persuasion effects taking place after mate choice rather than to assortative mating, the levels of similarity are probably too high to dismiss assortative mating entirely. This is confirmed by a preliminary look at the impact of controlling for assortative mating on these 28 attitude items. The Virginia 30K study includes data for parents of twins in the study, including parents' individual responses to the same W-P items that the twins responded to...The results for a partial correlation analysis controlling for parental agreement are reported in Table 2, on the row just below the results for the overall index. For MZ twins the issue of whether their parents agree or disagree on a particular item makes little difference (.65 without control versus .64 after partialing out the effect of parental agreement). In contrast, the correlation between DZ twins decreases modestly when the impact of parental agreement is removed (.43 without control versus .37 after partialing out the effect of parental agreement). Further, the tendency of assortative mating to deflate estimates of heritability while inflating estimates of the impact of shared environment is clear. Without controls, the estimate of heritability for the overall index is .43 and the average estimate of the impact of shared environment is .22. When the impact of parental agreement is partialed out, the average estimate of heritability rises to .53, and the average estimate of the impact of shared environment drops to .11."
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