how much of the total variation among them in a trait, suchas scores on cognitive skills tests, can be attributed to (1) vari-ations in genes (this estimate is called “heritability”); (2) thefamily environments they share; and (3) everything else(including measurement errors), categorized as unshared envi-ronments.

dentical twins separated at birth andbrought up in separate environments provide the ideal testcases, but such cases are exceedingly difficult to find

Career similarity between twins could be a resultof coincidence, genetic likeness, similar family influences, or acombination of any of these factors. A scholarly study may notbe as sensational,

However, with increasing research and public interest in genetics more attention has been paid to biological aspects of crime and to genetic variations within the normal range. Research has focussed on violent and antisocial behaviours which are criminal or may be seen as a precursor to criminal behaviour, for example, antisocial behaviour in young people.

Bearman considers the reasons why sociologists are concerned about genetic effects on behaviour; first they see it as legitimating existing societal arrangements, which assumes that ‘genetic’ is unchangeable. Second, if sociologists draw on genetic research it contaminates the sociological enterprise and, third, whatever claims are made to the contrary, it is a eugenicist project

hile researchers are aware of the complexity of gene-environment interaction, the ‘nature and nurture’ model persists as a simple way of framing discussion on the causes of behaviours.

Twin researchers assume that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share more than 50 percent of their genes

disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence.

Twin studies estimate the heritability of a trait, but molecular genetics attempts to pinpoint the effects of a particular gene.The future of twin research will involve combining traditional twin studies with molecular genetics research, according to Hewitt, who believes that day is already here.

40% of the differences among individuals could be attributed to their distinct genetic makeup, while 60% to environmental factors.

identical and fraternal twins

environmental factors more likely determine how much money proactive employees earn, while genetics more likely determine job satisfaction

It is the reciprocal relationship between people's dispositions and their work experiences that can make them more or less proactive

the interaction between the genetic and environmental factors determines why some employees are more proactive than others.

During the eugenics movement of the 1920s and 1930s, both in the United States and Europe, society became less, not more, tolerant of human variation and misfortune. Based on racial theories that held Eastern Europeans to be genetically inferior to Anglo-Saxon stock, Congress passed (and Calvin Coolidge signed) a 1924 law to restrict immigration, and by 1940 more than 30 states had laws permitting forced sterilizatio

What this means is that those seeking help for excessive drinking are told they have a disease (though the exact nature of the disease is unknown), that it's probably a genetic condition, and that the only treatment is abstinence.

reported strong evidence of genetic influence on antisocial personality disorder, but it also noted that many genes are probably involved. Getting from those unknown genes to an actual act of vandalism or assault--or a life of barbaric violence--requires at this point a monstrous leap of faith.

A whole branch of psychiatry known as "social psychiatry" was dedicated to helping the mentally ill by rooting out such pathogens as poverty and racism. There is no longer much evidence of these sensibilities at work today. NIMH now focuses its studies almost exclusively on brain research and on the genetic underpinnings of emotional illnesses.

On the personal level, a belief in the power of genes necessarily diminishes the potency of such personal qualities as will, capacity to choose, and sense of responsibility for those choices--if it's in your genes, you're not accountable.

But her most powerful advocacy tool by far is the PET scan. She takes a collection of these colorful brain images up to Capitol Hill to put on a show, giving lawmakers a window on a "broken" brain in action. "When they see that it's not some imaginary, fuzzy problem, but a real physical condition, then they get it: 'Oh, it's in the brain.' "

This cultural shift has political and personal implications. On the personal level, a belief in the power of genes necessarily diminishes the potency of such personal qualities as will, capacity to choose, and sense of responsibility for those choices--if it's in your genes, you're not accountable. It allows the alcoholic, for example, to treat himself as a helpless victim of his biology rather than as a willful agent with control of his own behavior. Genetic determinism can free victims and their families of guilt--or lock them in their suffering.

Given that we still see differences between them, this really has to be of prenatal origin

In those pre-birth control, pro-marriage times, female co-twins were 25 percent less likely to have children than female-female twins, raised smaller families and were, in fact, less likely to marry at all. Further, Lummaa's data showed that the result held steady across social classes and even if the male twin died within three months of birth, leaving the girl to be brought up as an only child.

She concluded that the socializing effect of a brother was as important, or more, than prenatal exposure to androgens in such cases.

"The nature of the chorion [the protective membrane around each fetus] is so different in humans and rodents," he says. "They have a much leakier system than ours. You have to think that would make a difference."

Further experiments confirmed that the physical difference could be traced to a prenatal diffusion of testosterone

The researchers discovered that for developing females, the sex of their pod-neighbor made a real difference.

Female is considered the "default" state for human development; without that extra testosterone, the body simply continues toward a female design. If XY males don't get enough prenatal androgens, as happens with some genetic defects, those males develop looking like well-formed females.

The timing and the amount of androgens such as testosterone before birth are essential to normal male development.

For a long time, people just thought that all the learning could be done in same-sex twins. So until recently they've been grossly understudied. And now, all at once, there's awareness of this tantalizing potential."

Their female co-twins had a lower level of eating disorders, tending toward the male range. By contrast, the same-sex female twins had the highest level of all the twin sets questioned. The Michigan State scientists suspect that the reason can be found in the prenatal environment: Sharing the space with a developing male can apparently alter female development in some small but interesting w

Galton's formulation was greatly assisted by Charles Darwin's theory of particulate inheritance. But where Darwin thought these particles ("gemmules") might sometimes allow acquired characteristics to be passed on, Galton was convinced otherwise. To him, they were unchanging--hard, like atoms--and, like many others, he thought particulate inheritance could do for biology what atomic theory had done for chemistry.

trying to determine how much of a trait is produced by nature/genes and how much by nurture/environment is as useless as asking whether the drumming we hear is made by percussionist or instrument.

Genetical science has outgrown the false antithesis between heredity and environment productive of so much futile controversy in the past." And everything we have learned since has only underscored the fact that the entanglement of developmental processes is from the start immensely intricate.

The unfortunate effect is to obscure the basic fact that the causes of the development of a trait are not separable.

The discovery of how genes actually influence human behaviour, and how human behaviour influences genes, is about to recast the debate entirely. No longer is it nature versus nurture, but nature via nurture. Genes are designed to take their cues from nurture."

the answer is neither nature nor nurture, but both

Sometimes the distinction is between what is inborn and what is acquired after birth; more often, it is between genes and environment. But not only does nurture affect prenatal development, we also need to ask what exactly is a gene, and what does it do?

Genetics research has at times generated heated ethical and political debates. For example, some authors have commented that studying the genetic components of traits will lead to a resurgence in the eugenics movement and, in particular, the misuse of heritability research on intelligence by some proponents of genetic engineering

Study found moderate to high correlations for processes specifically related to aging, such as memory decline. The conclusion reached in these studies is that genetics are responsible for trait similarity, and that environmental factors have little influence.

Twin studies are particularly used in clinical and psychological research aimed at attempting to find the genetic component of certain disorders, such as schizophrenia, as well as of intelligence and various personality traits.

The mathematical model that best explained the similarities and differences between twins, the team reports in the journal Nature, includes genetic effects and environmental effects, as every other model does--but also what the scientists call the "maternal effect." This effect of the uterine environment accounts for 20 percent of the IQ similarities between twins and 5 percent of the similarities between other siblings, they calculate. "The effects of genes are correspondingly reduced," conclude the authors, to 34 percent rather than the 70 percent calculated from twins studies.

fraternal twins raised together are more similar than nontwin siblings raised together, even though nonidentical twins are no more alike genetically than other siblings. "But it is not at all clear that this is because of the shared intrauterine environment," says behavioral geneticist Gregory Carey of the University of Colorado

the twin who was lighter at birth had a more adverse metabolic profile in adulthood compared to its genetically identical co-twin, who was heavier at birth [6, 7, 8, 9, 10]. This suggests that the association between low birth weight and increased T2D risk is at least partly independent of genetic factors.

These differing chemical tags may help explain why identical twins look slightly different, have their own personalities and may have different susceptibility to diseases

actors in the womb influencing fetus growth, such as size of the umbilical cord, may also create epigenetic discrepancies between twins. Identical twins sharing a placenta were more epigenetically different than twins who had their own;

So identical twins, which result when a single embryo splits, start out with the same epigenetic profile. Fraternal twins, the product of separate fertilization events, begin with slightly different epigenetic patterns, Petronis says.

Identical twins are born from a single fertilised egg, or zygote. Genetically speaking, therefore, they are indeed the same.

They uncovered a significant amount of variation between twins, possibly enough to explain why apparently heritable diseases that require the coincidence of several genetic risk-factors do not, in practice, always appear in both twins.

The results suggest that although monozygotic twins do differ epigenetically, they differ less than dizygotic twins.

That some methylation escapes pre-fertilisation erasure has been suggested by experiments on other animals, but this has been thought the exception, rather than the rule. If that were so, though, the degree of difference between identical and non-identical twins would be broadly the same. It is not.

Among identical twins, 80 percent of those surveyed reported feeling closer and more familiar with their twin than they did to their best friends, suggesting a strong genetic component in the bond between identical twins.

The Minnesota researchers found that about 70 percent of IQ variation across the twin population was due to genetic differences among people, and 30 percent was due to environmental differences.

"If, in this group of genes, the epigenetic markers are established before the embryo splits into two, then the markers will be the same in both twins," Waterland said. "In essence, both twins inherit an intimate molecular memory of their shared developmental legacy as a single individual. On the other hand, genes at which epigenetic markers are set after the embryo splits can have greater epigenetic differences between the two twins."

The findings, presented Friday (Nov. 9) here at the American Society of Human Genetics meeting, may partly explain why one twin gets cancer while another stays healthy. The study also suggests that these genetic changes are surprisingly common.

A higher percentage of disease incidence in both identical twins is the first indication of a genetic component. Percentages lower than 100% in identical twins indicates that DNA alone does not determine susceptibility to disease.

nature vs. nurture

"That twins were often dressed the same and treated exactly the same, she felt, interfered with their independent psychological development," Perlman says.

Wright says that no such study will ever be done again — nor should it. But he acknowledges that it would be very interesting to learn what this study has to teach us.