Genetische Grundlagen politischer Überzeugungen

In der Zeit findet sich ein interessanter Artikel zu den genetischen Grundlagen der politischen Überzeugung. Dass auf die politische Ausrichtung einen gewissen Rückhalt in der Biologie haben kann, hatte ich auch bereits in dem Artikel „Biologische Grundlagen der politischen Überzeugung“ dargestellt.

Aus dem Zeitartikel:

Einen Anfang machten Nicholas Martin, Lindon Eaves und Hans Eysenck 1986. Im Fachblatt PNAS veröffentlichten sie eine klassische Zwillingsstudie. Dabei machten sich die Forscher zunutze, dass eineiige und zweieiige Zwillinge zwar dasselbe Elternhaus und Umfeld haben, aber nur eineiige Zwillinge auch ihr gesamtes Erbgut teilen. Ähneln sich eineiige Zwillinge in einer bestimmten Eigenschaft im Schnitt mehr als zweieiige, so liegt das also an den Genen. Aus den Unterschieden der beiden Zwillingsgruppen in einer bestimmten Eigenschaft lässt sich errechnen, wie stark der Einfluss der Gene darauf ist. Die Eigenschaft, die Martin und Kollegen an 4.600 Zwillingspaaren aus Australien und Großbritannien untersuchten, war ihre politische Überzeugung, also etwa ihre Einstellungen zu Todesstrafe und Abtreibung, Gewerkschaften und Einwanderung. Im angelsächsischen Raum werden diese Einstellungen meist auf einer Skala Liberal-Konservativ verortet, was in Deutschland in etwa Links-Rechts entspricht. Das Ergebnis der Forscher war ein Tabubruch: Die Unterschiede in den politischen Ansichten seien etwa zur Hälfte auf die Gene zurückzuführen.

Also zur Hälfte auf die Gene, das ist schon ein recht deutlicher Wert. Ich kann mir gut vorstellen, dass bestimmte Gehirnausrichtungen biologisch sein können. Bestimmte Charaktereigenschaften und Persönlichkeitsfaktoren passen besser zu bestimmten politischen Vorstellungen und die „Big 5“ haben starke vererbbare Komponenten.

Widerstand gegen die Forschung zeigte sich schnell:

der Politikwissenschaftler John Hibbing von der Universität Nebraska. 2005 veröffentlichte er eine Studie, die die gleichen Zusammenhänge zwischen Genetik und Politik zeigte. Dieses Mal gab es ein Echo, wenn auch kein freundliches. „Die meisten Politikwissenschaftler fanden die Behauptung, Genetik könnte bei der politischen Orientierung eine Rolle spielen, absurd und sogar gefährlich“, sagt Hibbing. Er erhielt etliche anonyme E-Mails, die seine Arbeit schmähten. Viele Menschen schienen das Ergebnis instinktiv abzulehnen.

Ein bekanntes Problem. Biologie scheint den freien Willen auszuhebeln und an diesem Festzuhalten ist für viele Leute sehr wichtig.

Die weitere Forschung bestätigt die Ergebnisse aber:

Genetiker hingegen überraschte das Ergebnis kaum. Schließlich zeigen ihre Untersuchungen seit 40 Jahren, dass die Gene die Persönlichkeit eines Menschen prägen. Tatsächlich sind inzwischen zahlreiche Studien in Australien, Dänemark, Schweden und anderen Ländern zu ähnlichen Ergebnissen gekommen. Zuletzt erschien im September eine Analyse von Pete Hatemi und Rose McDermott in Nature Reviews Genetics. Darin haben sie alle Zwillingsstudien, die in den Jahren 1974 bis 2012 veröffentlicht wurden, zusammengefasst. Für die meisten politischen Themen fanden die Forscher einen großen genetischen Anteil.

Interessant auch die Forschung der beiden dazu, warum politische Einstellungen bei Paaren so häufig gleich sind:

Most social science research portrays attitudes and behaviors as a product of one’s environment or social upbringing. Recently, however, scholars have begun to expand upon this paradigm by showing that biological factors such as genes, which are passed from parents to offspring, can also help explain differences in political attitudes and behaviors. As a result, illuminating how spouses select one another is the first step toward understanding both the genetic and social transmission of political preferences from parents to offspring. Yet the question of whether individuals actively seek out mates who are more politically similar is unknown. To address this lacuna, data were gathered from Internet dating profiles. These data show that most individuals are reluctant to advertise politics when attempting to attract a mate. However, the correlates of political attitudes and behavior, such as education and civic engagement, do predict whether a person uses politics as a way to attract a mate. Thus, although spouses share such predilections more than almost any other trait, individuals do not appear to initially select potential dates along political lines.

Quelle: Do bedroom eyes wear political glasses? The role of politics in human mate attraction

Aus der Diskussion:

First, humans desire compatibility in their long-term relationships (Vandenberg 1972); from an evolutionary perspective, compatibility should increase the likelihood of a mate pair being able to successfully raise offspring (Geary et al., 2004). Accordingly, while we might not be choosy on politics at the outset of a relationship, individuals are likely to make a long-term association with a partner who shares critical values, such as their political attitudes. Thus, it is only the majority of those who share political views who remain in long-term relationships. Second, people could be making long-term mate choices on the basis of nonpolitical characteristics that correlate with political predilections, such as religiosity (Botwin, Buss, & Shackelford, 1997), physiology (Amodio, Jost, Master, & Yee, 2007) or intelligence (Kanazawa 2010; Mare 1991; Rushton & Nicholson 1988). Consequently, when one assorts on any of these nonpolitical characteristics, one is also likely to assort on politics, even if such a pairing is unintended

Wenn zwar offiziell politische Ansichten bei der Partnerwerbung nicht in der Vordergrund gestellt werden, aber inoffiziell oder zumindest aufgrund allgemeiner Gleichheit auch wieder eine wichtige Rolle spielen, dann würde es erklären, warum Personen, die in einer Partnerschaft sind, häufig in ihren politischen Ansichten gleicher sind.

Gleichzeitig würde es auch erklären, warum Kinder häufig eine ähnliche politische Einstellung wie ihre Eltern hätten.

Der Zeit-Artikel zum Einwand der Determiniertheit:

Ohnehin behauptet kein Forscher, dass es ein Gen für Konservatismus oder Sozialdemokratie gibt. Vermutlich haben Hunderte oder Tausende Gene einen Einfluss auf unsere politischen Überzeugungen. Und diese Gene geben dem Individuum lediglich eine Prädisposition mit, sie beeinflusst ein wenig, wie der Mensch die Welt sieht, wie er auf sie reagiert. Der Rest passiert ganz von allein.

Wiederum geht es also um eine Prädisposition, nicht um Determiniertheit. Hier können sich die Lebensumständen natürlich wieder auswirken

Es gibt verschiedene Theorien zu bestimmten Ursachen:

So sind zum Beispiel Menschen, die eine bestimmte Variante des Dopaminrezeptors D4 tragen und viele Freunde haben, liberaler als der Durchschnitt. Menschen, die diese Genvariante tragen, das ist aus anderen Untersuchungen bekannt, suchen häufiger nach neuen Erfahrungen. Zusammen mit einem großen sozialen Umfeld könnte das dazu führen, dass sie mehr verschiedene Erfahrungen machen, was wiederum dazu führen könnte, dass sie liberale Überzeugungen entwickeln. Andere Studien haben andere Botenstoffe im Gehirn wie NMDA, Serotonin oder Glutamat mit liberalen oder konservativen Überzeugungen verknüpft.

Hier scheint also noch einiges an Forschung erforderlich zu sein.

Und schließlich noch die evolutionäre Herleitung:

Was hat die Biologie mit der Ideologie zu tun? Sehr viel, sagen Psychologen. Bei vielen politischen Fragen unserer Zeit geht es um dieselben Themen, mit denen sich schon unsere Vorfahren beschäftigen mussten: Fortpflanzung, Verteidigung, Kooperation, Überleben. So berührt Einwanderungspolitik die uralte Frage, wen wir als Teil unserer Gruppe akzeptieren, soziale Sicherungssysteme werfen die Frage auf, wie Ressourcen am besten aufgeteilt werden, und Außenpolitik beschäftigt sich maßgeblich mit dem Schutz der eigenen Gruppe gegen andere Gruppen. Jonathan Haidt hat fünf moralische Fundamente ausgemacht, auf denen unsere Werturteile gründen: Fürsorge, Fairness, Loyalität, Autorität und Reinheit. Für jedes dieser Module sieht Haidt gute evolutionäre Gründe. So habe die Evolution Frauen und Männer begünstigt, die der Anblick eines leidenden Kindes berührt und zum Handeln animiert (Fürsorge). Ein Gerechtigkeitsempfinden sei wichtig gewesen, um in einer kooperativen Gemeinschaft nicht über den Tisch gezogen zu werden (Fairness) und Ekel vor Körperflüssigkeiten oder bestimmten Tieren habe Menschen davor geschützt, sich mit einer Krankheit anzustecken (Reinheit). Für Haidt sind die fünf Kategorien so etwas wie die Primärfarben unserer Moral. Aus ihnen lassen sich die verschiedensten Charaktere mischen – und offenbar ist genau das bei Liberalen und Konservativen der Fall. Während Liberale ihre politischen Überzeugungen vor allem aus Fairness und Fürsorge ableiten, werten Konservative alle fünf Module etwa gleich stark.

Zwillinge und Homosexualität

Eine Studie dazu, welchen Anteil Umwelt bzw. Gene an Homosexualität haben anhand einer Zwillingsstudie:

There is still uncertainty about the relative importance of genes and environments on human sexual orientation. One reason is that previous studies employed selfselected, opportunistic, or small population-based samples. We used data from a truly population-based 2005–2006 survey of all adult twins (20–47 years) in Sweden to conduct the largest twin study of same-sex sexual behavior attempted so far. We performed biometric modeling with data on any and total number of lifetime same-sex sexual partners, respectively. The analyses were conducted separately by sex. Twin resemblancewas moderate for the 3,826 studied monozygotic and dizygotic same-sex twin pairs. Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance, the shared environment .00, and the individualspecific environment .61–.66 of the variance.Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and 64–.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.

Quelle: Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden

Aus der weiteren Zusammenfassung:

It has been suggested that individual differences in heterosexual and homosexual behavior result from unique environmental factors such as prenatal exposure to sex hormones, progressive maternal immunization to sex-specific proteins, or neurodevelopmental instability (Rahman, 2005). Although the unique environmental variance component also includes measurement error, the present results support the notion that the individual-specific environment does indeed influence sexual preference. In conclusion, although confidence intervals were wide, we believe this study provides the most unbiased estimates presented so far of genetic and non-genetic contributions to same-sex sexual behavior. The results should inform further research on this complex trait

Und eine weiter Studie, die auf die Digit Ratio abstellt:

The 2nd to 4th digit ratio has been argued to reflect prenatal hormonal influences and is reportedly associated with various psychological and behavioral traits, such as sexual orientation, cognitive abilities, and personality. We examined genetic and environmental influences on the 2nd to 4th digit ratio (2D:4D) using a Japanese twin sample (N = 300). The genetic analysis showed substantial additive genetic influences for both right and left hand 2D:4D. The rest of the variance was explained mainly by environmental influences not shared within twin pairs. These findings were, in general, in accordance with preceding studies with primarily Caucasian twin samples. The bivariate genetic analysis revealed that the additive genetic influences were largely shared between the right and left hand, while the non-shared environmental influences were largely unique to each hand. Results from a comparison of opposite-sex and same-sex twins were not significant, although they were in the predicted direction according to the prenatal hormone transfer hypothesis. Female monozygotic twin pairs discordant in sexual orientation showed significant within-pair differences in left hand 2D:4D, where non-heterosexual twins had lower (more masculinized) 2D:4D. In addition, we found that non-heterosexual male MZ twins had larger (more feminized) 2D:4D than their heterosexual co-twins. These results suggest the existence of non-shared environmental influences that affect both 2D:4D and sexual orientation

Quelle: The Second to Fourth Digit Ratio (2D:4D) in a Japanese Twin Sample: Heritability, Prenatal Hormone Transfer, and Association with Sexual Orientation

Aus der Studie:

If the prenatal environment has some influence on 2D:4D, we would expect that even DZ co-twins are similar to each other because of the shared prenatal environment. The small shared environmental influences do not support the prediction. This suggests the possibility that twins do not share their prenatal hormonal environment. To test prenatal hormonal influences more directly, a second type of twin study compared twins from opposite-sex pairs (OS pairs) to those from same-sex pairs (SS pairs).(…) A recent meta-analysis showed that non-heterosexual females, including both homosexuals and bisexuals, had smaller (more masculine) 2D:4D than heterosexual females (Grimbos et al., 2010). Hall and Love (2003) conducted a within-pair comparison of 2D:4D of seven female MZ twin pairs with discordant sexual orientation. In each pair, one co-twin had a heterosexual orientation and the other had a homosexual orientation. In agreement with Grimbos et al.’s meta-analysis, Hall and Love reported that female homosexual MZ co-twins had more masculine 2D:4D compared with their heterosexual co-twins. A within-pair comparison of five female MZ twin pairs with concordant sexual orientation was not significant. Because MZ pairs share 100 percent of their genotypes, the differences between them must be explained by non-shared environmental influences. The environmental influences could include epigenetic effects. Previous results suggest that the same non-shared environmental factors that affect 2D:4D could also affect sexual orientation, indicating an overlap of environmental effect between the two phenotypes.

Dazu hatte ich auch bereits etwas in anderen Artikeln geschrieben:

Die Studien zeigen, dass die Gene und die pränatalen Hormone Gründe für Homosexualität haben können und Unterschiede zwischen Zwillingen gerade in diesem Bereich bestehen.

Zwillingsstudien in der Verhaltensforschung und Kultur / Gesellschaft

Ein Artikel von Turkheimer zur Verhaltensforschung, der auch das Zusammenspiel von Natur und Gesellschaft behandelt:

Zunächst relativ generell:

The presence of genetic influences does not mean that genes “cause” behavior in any preordained way. Rather, it means that genes predispose toward some (tacit or active) ongoing series of individual behavioral choices. These choices have effects on later circumstances that affect later options for genetically influenced behavioral choices, and so on. This causal chain is captured in the behavior-genetic literature through the concept of gene–environment correlation, or genetically influenced differences in environmental exposure. Gene–environment correlation may be passive, active, or evocative (Plomin, De-Fries, McClearn, & McGuffin, 2008). It is passive when, for example, children receive both genes influencing antisocial behavior and abusive treatment from their parents and model the behavior they receive. It is active when people genetically inclined to be social seek social groups in which to participate and avoid spending long periods of time alone. It is evocative when children with genetically influenced difficulties with emotional control throw temper tantrums that generate angry responses from their parents. Gene–environment correlation is described in the developmental psychopathology literature as social selection, in which the association between risk and outcome reflects origin of risk in the individual rather than the effects of risk. For example, poverty may be a risk factor for schizophrenia because people at genetic risk for schizophrenia tend to drift into poverty because of inability to maintain educational and occupational performance, as well as because poverty provides the disease-triggering stress. In the epidemiological literature, gene–environment correlation is described as reverse causation or confounding, in which some to all of the association between an experience and an outcome reflects the effects of people on their environments rather than the effects of their environments on them.

Eine interessante Darstellung, aus der man sehen kann, wie sich die beiden Faktoren gegenseitig beeinflussen können und wie schnell auch  biologische Ursachen mit sozialen Ursachen verwechselt werden können.

Die Zusammenfassung dort finde ich auch noch ganz interessant:

The discovery that all behavior is partially heritable transformed psychology, but, ironically, it also transformed behavior genetics. Once we accept that basically everything—not only schizophrenia and intelligence, but also marital status and television watching—is heritable, it becomes clear that specific estimates of heritability are not very important. The omnipresence of genetic influences does not demonstrate that behavior is “less psychological” or “more biologically determined” than had originally been thought; rather it shows that behavior arises from factors intrinsic as well as extrinsic to the individual. The real implications of heritability lie not in questions of relative biological determinism but in revealing the need to understand both the mechanisms through which the individual, whether consciously or not, directs his or her own life course and his or her power to do so. In psychology, where it is not ethically possible to conduct randomized experiments on life outcomes, the natural experiment provided by the twin study can be most helpful in addressing these issues.

Er hat in dem Bericht einige interessante Beispiele für Zwillingsstudien. Lesenswert.

Wie beeinflussen sich verschiedengeschlechtliche Zwillinge im Mutterleib? (Twin testosterone transfer hypothesis)

Eine Hypothese ist, dass bei verschiedengeschlechtlichen Zwillingen gegenseitige hormonelle Beeinflussungen auftreten können. Insbesondere wird vermutet, dass der höhere Testosteronspiegel des männlichen Zwillings den weiblichen Zwilling beeinflusst. Wenn der weibliche Zwilling einem höheren Testosteron ausgesetzt ist, dann könnte das in den passenden Monaten Auswirkungen haben. Es ist aber in diesem Bereich noch vieles unklar:

Die Wikipedia hat folgendes dazu:

Prenatal Testosterone Transfer also known as prenatal androgen transfer or prenatal hormone transfer refers to the phenomenon in which testosterone synthesized by a developing male fetus transfers to one or more developing fetuses within the womb and influences development. This typically results in the partial masculinization of specific aspects of female behavior, cognition, and morphology,[1] though some studies have found that testosterone transfer can cause an exaggerated masculinization in males.[2] There is strong evidence supporting the occurrence of prenatal testosterone transfer in rodents and other litter-bearing species,[1] such as pigs.[3] When it comes to humans, studies comparing dizygotic opposite-sex and same-sex twins suggest the phenomenon may occur, though the results of these studies are often inconsistent.[4][5]

Der Mechanismus wird dort wie folgt dargestellt:

Testosterone is a steroid hormone; therefore it has the ability to diffuse through the amniotic fluid between fetuses.[6] In addition, hormones can transfer among fetuses through the mother’s bloodstream

Eine Studie dazu:

In this paper we review the evidence that fetuses gestated with a male co-twin are masculinized in development, perhaps due to the influence of prenatal androgens: the so-called twin testosterone transfer (TTT) hypothesis. Evidence from studies of behavioral, perceptual, cognitive, morphological and physiological traits in same- and opposite-sex human twins is considered. Apart from two studies reporting increases in aspects of sensation-seeking for females with a male rather than a female co-twin, there is sparse evidence supporting the TTT hypothesis in behavioral studies. Outcomes from studies of perception (in particular otoacoustic emissions) and cognition (in particular vocabulary acquisition and visuo-spatial ability) provide more consistent evidence in support of masculinized performance in twins with a male co-twin compared to twins with a female co-twin. The outcomes favorable to the TTT hypothesis for otoacoustic emissions and visuo-spatial ability are restricted to females. Studies of physiology and morphology (e.g., brain volume, tooth size and 2D:4D ratio) also show some influence of co-twin sex, but again these effects are often restricted to female twins. Because females produce little endogenous testosterone, the effects of gestation with a male co-twin may be more pronounced in females than males. Thus, while uneven, the evidence for the TTT hypothesis is sufficient to warrant further investigation, ideally using large samples of same- and opposite-sex twins, along with control groups of same- and opposite-sex siblings when the characteristics assessed are potentially open to social influences.

 

Highlights

► Hormones can transfer between fetuses in animals affecting phenotypic development. ► The existence of a similar mechanism in humans remains uncertain. ► A review finds little evidence for masculinization of behavior due to testosterone transfer. ► Some evidence consistent with transfer for perception, cognition and physiology. ► Evidence of masculinization is typically restricted to females with a male co-twin.

Quelle: Evaluating the twin testosterone transfer hypothesis: A review of the empirical evidence

Hier ist also noch vieles unklar. Es scheint einige Hinweise zu geben, dass der Mechanismus auch bei Menschen greift, aber es ist hier wohl weitere Forschung abzuwarten.

„Was ist so schlimm an einem Jungen, der ein Kleid tragen möchte“ (Gendervarianz)

In dem Bericht „What’s So Bad About a Boy Who Wants to Wear a Dress?„, aus dem Dr. Mutti ihre Passage über Tomboys hatte findet sich noch einiges Interesantes zu Jungs, die eine Gender Varianz haben:

When Alex was 4, he pronounced himself “a boy and a girl,” but in the two years since, he has been fairly clear that he is simply a boy who sometimes likes to dress and play in conventionally feminine ways. Some days at home he wears dresses, paints his fingernails and plays with dolls; other days, he roughhouses, rams his toys together or pretends to be Spider-Man. Even his movements ricochet between parodies of gender: on days he puts on a dress, he is graceful, almost dancerlike, and his sentences rise in pitch at the end. On days he opts for only “boy” wear, he heads off with a little swagger. Of course, had Alex been a girl who sometimes dressed or played in boyish ways, no e-mail to parents would have been necessary; no one would raise an eyebrow at a girl who likes throwing a football or wearing a Spider-Man T-shirt.

Es scheint also eine Veranlagung mit wechselnden Geschlechterrollen zu sein. Auch schwer in soziale Theorien einzuordnen.

Etwas zum Forschungsstand wird dort wie folgt zusammengefasst:

Relatively little research on gender-nonconforming children has been conducted, making it impossible to know how many children step outside gender bounds — or even where those bounds begin. Studies estimate that 2 percent to 7 percent of boys under age 12 regularly display “cross-gender” behaviors, though very few wish to actually be a girl. What this foretells about their future is hard to know. By age 10, most pink boys drop much of their unconventional appearance and activities, either because they outgrow the desire or subsume it. The studies on what happens in adulthood to boys who strayed from gender norms all have methodological limitations, but they suggest that although plenty of gay men don’t start out as pink boys, 60 to 80 percent of pink boys do eventually become gay men. The rest grow up to either become heterosexual men or become women by taking hormones and maybe having surgery. Gender-nonconforming behavior of girls, however, is rarely studied, in part because departures from traditional femininity are so pervasive and accepted. The studies that do exist indicate that tomboys are somewhat more likely than gender-typical girls to become bisexual, lesbian or male-identified, but most become heterosexual women.

Und zum Umgang mit davon betroffenen Kindern:

There is also little to no evidence that therapeutic interventions change the trajectory of a child’s gender identification or sexual orientation. Clinicians who oppose traditional treatments contend that significant gender nonconformity is akin to left-handness: unusual but not unnatural. Rather than urging children to conform, they teach them how to respond to intolerance. They encourage parents to accept their children’s gender expression, especially because studies show that parental support helps to inoculate gender-atypical children against ostracism and deflated self-esteem.

Wenn sie sind, wie sie sind, weil es irgendwie auf biologische Vorgänge zurückzuführen ist, dann wird auch hier eine Umerziehung wenig bringen. Es scheint dann in der Tat besser, sie einfach sein zu lassen, wie sie sein wollen.

Auch die kurze Abhandlung zu möglichen Gründen ist interessant:

No one knows why most children ease into their assigned gender roles so effortlessly and others do not. Hormone levels might play a role. One hint is provided by a rare genetic condition known as congenital adrenal hyperplasia, or C.A.H. The condition produces high levels of androgens, including testosterone, early in gestation, and can create somewhat male-like genitalia in genetic females. Girls with C.A.H. are typically raised as females and given hormones to feminize, yet studies show they are more physically active and aggressive than the average girl, and more likely to prefer trucks, blocks and male playmates. Though most turn out to be heterosexual, women with C.A.H. are more likely to be lesbian or bisexual than women who weren’t bathed in prenatal androgen.

Genetics might also be a factor in gender expression. Researchers have compared the gendered behavior of identical twins (who share 100 percent of their genes) with that of fraternal twins (who share roughly half). The largest study was a 2006 Dutch survey of twins, 14,000 at age 7 and 8,500 at age 10. The study concluded that genes account for 70 percent of gender-atypical behavior in both sexes. Exactly what is inherited, however, remains unclear: the specific behavior preferences, the impulse to associate with the other gender, the urge to reject limits imposed on them — or something else entirely.

Ich gehe davon aus, dass es sich dabei um die folgende Studie handelt:

The aim of this study was to investigate the prevalence of cross-gender behavior during childhood, to estimate the influence of genotype and environment on variation in cross-gender behavior, and to explore the association of cross-gender behavior with maternal ratings of behavior problems as indexed by the Internalizing and Externalizing scales of the Child Behavior Checklist (CBCL). Cross-gender behavior was assessed by two items from the CBCL: “behaves like opposite sex” and “wishes to be of opposite sex.” As part of an ongoing longitudinal study of the Netherlands Twin Registry, mothers were asked to complete the CBCL for their twins when they were 7 (n∼14,000 twins) and 10 years old (n∼8,500 twins). The prevalence of cross-gender behavior (as measured by maternal report of behaving like or wishing to be the opposite sex) was 3.2% and 5.2% for 7-year-old boys and girls, respectively, and decreased to 2.4% and 3.3% for 10-year-old boys and girls. Surprisingly, the prevalence rate of cross-gender behavior of girls with a male co-twin was lower than of girls with a female co-twin. At both ages, the similarity for cross-gender behavior was greater in monozygotic than in dizygotic twins pairs. Genetic structural equation modeling showed that 70% of the variance in the liability of cross-gender behavior could be explained by genetic factors, at both ages and for both sexes. Cross-gender behavior was associated with higher scores on Internalizing and Externalizing problems, both in boys and in girls.

Quelle: Genetic and Environmental Influences on Cross-Gender Behavior and Relation to Behavior Problems: A Study of Dutch Twins at Ages 7 and 10 Years

70% durch die Gene, das ist ein sehr hoher Wert. Ich bin mal gespannt, was sich zu diesem Thema noch ergibt.

Eigentlich interessant, dass Dr. Mutti diesen Teil anscheinend eher uninteressant fand.

Biologische Unterschiede zwischen Zwillingen

Interessanterweise sind auch eineiige Zwillinge nicht vollkommen gleich.

Hier ein paar Punkte, die sie unterscheiden:

Aus einem Slateartikel:

That identical twins do not, in fact, have identical DNA has been known for some time. The most well-studied difference between monozygotic twins derives from a genetic phenomenon known as copy number variations. Certain, lengthy strands of nucleotides appear more than once in the genome, and the frequency of these repetitions can vary from one twin to another. By some estimates, copy number variations compose nearly 30 percent of a person’s genetic code.

These repeats matter. More than 40 percent of the known copy number variations involve genes that affect human development, and there are strong indications they explain observed differences between monozygotic twins. For example, it’s often the case that one identical twin will end up victimized by a genetically based disease like Parkinson’s while the other does not. This is probably the result of variations in the number of copies of a certain piece of DNA. Copy number variations are also thought to play a role in autism spectrum disorder, schizophrenia, and ADHD, all of which can appear in only one member of a monozygotic twin pair (PDF). If copy number variations can affect discrete and diagnosable disorders, then why shouldn’t they influence far more complex behaviors like your inclination to head to the polls on a Tuesday night in November?

That’s just the beginning of the genetic differences between monozygotic twins. As a result of mutations during development, about one in 10 human brain cells has more or less than the typical two copies of a chromosome. Identical twins also have different mitochondrial DNA, the genetic information stored in the cellar organelle responsible for processing glucose. Research suggests that mitochondrial DNA affects brain size among a host of other neurological traits.

Oder bei Steven Pinker, The Blank Slate:

Life is a pinball game in which we bounce and graze through a gantlet of chutes and bumpers. Perhaps our history of collisions and near misses explains what made us what we are. One twin was once beaten up by a bully, the other was home sick that day. One inhaled a virus, the other didn’t. One twin got the top bunk bed, the other got the bottom bunk bed. We still don’t know whether these unique experiences leave their fingerprints on our intellects and personalities. But an even earlier pinball game certainly could do so, the one that wires up our brain in the womb and early childhood. As I have mentioned, the human genome cannot possibly specify every last connection among neurons. But the “environment,” in the sense of information encoded by the sense organs, isn’t the only other option. Chance is another. One twin lies one way in the womb and stakes out her share of the placenta, the other has to squeeze around her. A cosmic ray mutates a stretch of DNA, a neurotransmitter zigs instead of zags, the growth cone of an axon goes left instead of right, and one identical twin’s brain might gel into a slightly different configuration from the other’s.67 We know this happens in the development of other organisms. Even genetically homogeneous strains of flies, mice, and worms, raised in monotonously controlled laboratories, can differ from one another. A fruit fly may have more or fewer bristles under one wing than its bottlemates. One mouse may have three times as many oocytes (cells destined to become eggs) as her genetically identical sister reared in the same lab. One roundworm may live three times as long as its virtual clone in the next dish.

Zu der Versorgung über die Nabelschnur und mögliche Auswirkungen  auch noch eine andere Studie:

The second to fourth finger digit ratio (2D:4D ratio) is a sex-dimorphic characteristic in humans that may reflect relative levels of first trimester prenatal sex hormones. Low interdigital ratio has been associated with high levels of androgens. It has been reported in unrelated women that low 2D:4D ratio is associated with lesbian sexual orientation, but because of the nature of those samples, it was not possible to conclude whether lower ratio (and hypothetically, higher androgen levels) in lesbians are due to differences in genetics as opposed to differences in environment. To test the hypothesis that low 2D:4D in lesbians is due to differences in environment, interdigital ratio data were analyzed in a sample of female monozygotic (MZ) twins discordant for sexual orientation (1 twin was lesbian, the other was heterosexual; n = 7 pairs). A control group of female MZ twins concordant for sexual orientation (both twins were lesbian) was used as a comparison (n = 5 pairs). In the twins discordant for sexual orientation, the lesbian twins had significantly lower 2D:4D ratios on both the right and left hands than their heterosexual cotwins. There were no significant differences for either hand in the twins concordant for sexual orientation. Because MZ twins share virtually the same genes, differences in 2D:4D ratio suggest that low 2D:4D ratio is a result of differences in prenatal environment.

Quelle: Finger-Length Ratios in Female Monozygotic Twins Discordant for Sexual Orientation

Das ist natürlich eine sehr kleine Studie, ich sehe sie insofern eher als ersten Ansatz, der aber zeigt, dass der Anteil der biologischen Faktoren bei der sexuellen Identität auch über die in anderen Studien ermittelten ca. 50% hinausgehen kann.

Vererbbare Faktoren in der sexuellen Orientierung bei Frauen

Eine Studie zu vererbbaren Faktoren in der sexuellen Orientierung bei Frauen:

Homosexual female probands with monozygotic cotwins, dizygotic cotwins, or adoptive sisters were recruited using homophile publications. Sexual orientation of relatives was assessed either by asking relatives directly, or, when this was impossible, by asking the probands. Of the relatives whose sexual orientation could be confidently rated, 34 (48%) of 71 monozygotic cotwins, six (16%) of 37 dizygotic cotwins, and two (6%) of 35 adoptive sisters were homosexual. Probands also reported 10(14%) nontwin biologic sisters to be homosexual, although those sisters were not contacted to confirm their orientations. Heritabilities were significant using a wide range of assumptions about both the base rate of homosexuality in the population and ascertainment bias. The likelihood that a monozygotic cotwin would also be homosexual was unrelated to measured characteristics of the proband such as self-reported history of childhood gender nonconformity. Concordant monozygotic twins reported similar levels of childhood gender nonconformity.

Quelle: Heritable Factors Influence Sexual Orientation in Women

Also die Zahlen noch mal kurz aufgeschlüsselt:

  • Eineiige Zwillinge: 48%
  • Zweieige Zwillinge: 16%
  • adoptierte Schwestern: 6%
Es wird also deutlich, dass bei mehr genetischer Übereinstimmung die Chancen steigen, dass die sexuelle Orientierung bei beiden homosexuell ist.
Die adoptierten Schwestern liegen im Schnitt der Homosexualität, insofern scheint der Erziehungsanteil gering zu sein.
Vgl. auch: