Though they have received much criticism, the advancement of statistical techniques (such as structural equation modelling) and the implementation of additional controls have allayed some of the concerns, if not all. The original twin study design has expanded to include studies of twins' extended families, longitudinal studies, and other variations. Some of these variations may allow researchers to address previous limitations also. Many molecular genetic studies have shown the usefulness of twin studies as an exploratory tool, whether or not the assumptions of equal environments and assortative mating are exactly met.

Findings from twin studies are often misunderstood, misinterpreted, and blown out of proportion, not just by the media, but even by serious scientists who get their work published.

Though lot of changes happened in the field of genetics over time, twin studies today are also based on the same assumptions that were made back in 1920s. Many of these are deeply flawed.

Finally, as the twin registries contain longitudinal data on large samples, they can therefore be used for conventional epidemiological analyses disregarding twinship status. Several studies have been performed on the association between exposure and outcomes using the registries as a population-based cohort or as the basis for nested case-control studies. When using twin data for these types of studies, the dependency between the twins in a pair should be taken into account by using generalized linear models or other techniques.

If analyses with external controls show associations between exposure and disease and the relative risk remains similarly high in the within-pair (co-twin) analyses, it speaks in favor of a causal effect of the exposure on the disease. On the other hand, if the relative risk is not increased in the within-pair comparisons (but only in the first-step analyses with external comparisons), this indicates that environmental factors early in life (for example, fetal environment, maternal smoking, or childhood socioeconomic status (SES)) are responsible for the initially observed findings.

The total variance in the trait can be partitioned into genetic variance, common environmental variance including shared (familial) environmental variance, and unique environmental variance.

Concordance rate (CR) for a disease or trait among identical and fraternal twin pairs is actually a statistical measure of probability: If one twin has a specific trait or condition, what is the probability that the other twin has (or will develop) that same trait or disease? Historically, CRs are computed separately for monozygotic (MZ) and dizygotic (DZ) pairs. When MZ concordances are greater than DZ concordances, genetic influences are indicated.(37)

It is assumed that genes and the environment typically make only separate and distinct contributions to a trait. In general, it is also assumed that only one type of genetic mechanism—usually additive—operates for a particular trait. However, traits can be inherited through different genetic mechanisms. Additive genetic mechanisms mix together the effects of each allele. For example, if genes for curly hair were additive, a curly-haired father and a straight-haired mother might have a child who has wavy hair.

Some assumptions are also made in twin studies; one of them is the assumption of random mating, which assumes 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 a greater percentage of their genes than expected. In the case of nonrandom mating, fraternal twins would have more genetically influenced traits in common than expected because the genes they receive from their mothers and fathers would be similar to each other.

Furthermore, identical twins reared apart were eerily similar to identical twins reared together in various measures of personality, personal mannerisms, expressive social behavior, and occupational and leisure-time interests. However, they did not find outstanding similarities between identical twins on measures such as standardized personality tests. Still, Bouchard's findings can be interpreted as strong support for genetic influences on personality.

Twin studies provide a strong basis for exploring the importance of any potential risk factors on a trait or condition by controlling the genetic variations. It has been one of the favorite research tools of behavioral geneticists and psychologists since long, mainly utilized to estimate the heritability of traits and to quantify the effect of a person's shared environment (family) and unique environment (the individual events that shape a life) on a trait.

In conclusion, twin studies of children providesupport for the hypothesis that genetic factors play asignificant role in OC manifestation. Twin studies ofadults are indicative, but a large twin study using abiometrical approach with continuous data is neededto provide conclusive evidence.

cross age groups and cultures,the best fitting model indicated additive genetic influ-ences of the CBCL OCS score between 45% and 61%,and unique environmental influences between 42% and55%. Only the NTR sample aged 12 years showedshared environmental influences of 16%. Minor sexdifferences were seen in the mixed-age MOTWIN twinsonly. No evidence of dominance, sibling interaction, orrater-contrast effects was seen

Recently Hudziak et al. (2004) examined 4246 twinpairs of the Netherlands Twin Register (NTR) and1461 twin pairs from the Missouri Twin Study Sample(MOTWIN

For OC behavior there was sub-stantial genetic influence with a heritability estimate of65% and a 35% estimate of nonshared environment.

First, they only selected a subset of items from thePadua Inventory, reducing the ability to detect a morecomplex and stable structure and increasing errorvariance. In a cross-sectional design, error variancecannot be distinguished from individual-specific envi-ronment with an underestimation of the impact ofgenetic factors as a result. Second, the study includedonly women, so conclusions cannot be generalized tomen. Third, the data exhibited a pronounced rightskew, although similar heritability estimates were pro-duced after correction

By using SEM, the best-fit modelsuggested heritabilities of 33% and 26% for obses-siveness and compulsiveness respectively. Uniqueenvironmental effects accounted for 67% and 74% ofthe variance

However, this approach cannot accommodate theeffect of sex on variances and covariance within andbetween twin pairs, nor can results easily be extendedto multivariate and longitudinal data

The heritability estimates for obsessional traitsand symptoms were 44% and 47% respectively. Noeffect of common environment was found, thus uniqueenvironment explained the remaining variation.

orgersenhypothesized that in our society, a possible genetic coremay perhaps be masked by the overwhelming environ-mental influence

OCD is in this case viewed as the equivalent ofextreme scores on symptom or trait measures. Such adimensional approach removes the problem ofscarcity of twins with the full disease and alsoremoves the need for population-based prevalencerates for comparison

Lastly, in population-basedsamples, the low prevalence of DSM-diagnosed OCDgenerally will lead to low statistical power to ascribethe familial clustering of OCD to either shared genesor shared environment

Although the useof DSM-III or DSM-III-R criteria reduces the risk offalse positive diagnoses, the failure to use separateinterviewers for each member of a twin pair with eachinterviewer blind to the zygosity status of the pairintroduces a large potential for inadvertent bias in thedetection of illness

In the co-twin group of the anxiety probands,two MZ twins with OCD were found but it is notclear in the article if these are concordant MZ twinsor not.

When the sexes were combined, theconcordance for anxiety disorders in the probandgroup labeled ‘all anxiety disorders without generalanxiety disorder (GAD)’ was 45% in MZ pairs to15% in DZ pairs. The author concludes that geneticfactors appear to influence the development of anxietydisorders in general, with the exception of GAD

Torgersen (1983) investigated genetic factors in thedetermination of six anxiety disorders in a study of 32MZ and 53 DZ adult same-sex twin pairs fromNorway.

Concordance rates for ‘obsessive symptoms or fea-tures with or without treatment’ were 87% for MZpairs and 47% for DZ pairs

The authors conclude thathead injury is a probable contributor to the develop-ment of OC neurosis in some cases

The standardization of thediagnosis and higher reliability of the zygosity determi-nation diminish some limitations of the case studiesdescribed abov

From the outset, however, it was recognized thatthe primary strength of this descriptive approach wasits ability to improve communication among cliniciansand researchers, not its established validity (Kupfer etal., 2002).

Finally, in many cases the method of zygositydetermination is unclear or there is lack of informa-tion to definitively establish monozygosity. With theselimitations in mind, no conclusions on the heritabilityof OCD can be drawn from this literature.

Parker (1964) describedtwo MZ twins discordant for OCD, attempting toillustrate with these cases that marked identificationcan still occur without both twins developing symp-toms of neurotic illness, throwing doubt on thevalidity of this purely environmental theory

This bias is caused by collecting twinsin an unsystematic way, which tends to favor MZ andconcordant pairs (Clifford et al., 1984). Anotherproblem with case studies was mentioned by Lewis(1935), who wrote that ‘a striking concordance in oneor two pairs of MZ twins proves nothing: one needs aseries and control group of fraternal twins’

Most studies of this era have failed to distinguishbetween OC neurosis and mixed neurosis, and showeda tendency to confuse OC neurosis with OC personal-ity or obsessive traits (Hoaken & Schnurr, 1980). Thehistory of OCD can partly explain this observation.

Clear defini-tions of obsessive neurosis or obsessive personalitydisorder did not exist and this is reflected in the differ-ent diagnostic information provided by the casestudies. Most studies provide insufficient clinical datato verify a diagnosis of OCD, severely hamperingjudgment on whether the subjects would meet currentstandardized diagnostic criteria

doption studies are generally rare,and to our knowledge no such studies have been pub-lished on OCD.

Family studies of OCD have suggested thatOCD is familial (Grados et al., 2003), which is not syn-onymous with heritable

The lifetime prevalence of OCD is estimatedbetween 0.7% and 2.5%

Obsessive–compulsive (OC) symptoms are remarkablydiverse and the clinical presentation can vary bothwithin and across patients over time

but a large twin study using a bio-metrical approach with continuous data is still neededto provide conclusive evidence.

It is concluded that only thestudies using the last method have convincinglyshown that, in children, obsessive–compulsive (OC)symptoms are heritable, with genetic influences in therange of 45% to 65%.

Twinstudies of OCD have a long history, starting in 1929

Our results suggest an apparent reduction with age in the influence of genetic factors on some but not all serum lipids. The early rearing environment appears to remain important in influencing total cholesterol levels later in life but seems less important in influencing other serum lipid and apolipoprotein levels in our elderly sample.

Evaluation of the estimates for cholesterol, as well as the hierarchical multiple regression analyses, suggests that there are age differences in heritability, although model-fitting analyses do not support this conclusion

However, they are consistent with normative data from Sweden and other Northern European countries

For total cholesterol, a shared rearing environment appeared to be more important among the older twins than the younger twins, and more important among women than men.

Because the total variances for the lipid values of the sexes also differed, we reformulated these models, basing them on intraclass correlations, which are not affected by differences in variances.

Most of the correlations for the younger monozygotic twins were higher than those for the older monozygotic twins, suggesting greater heritability among the younger twins. This pattern was most apparent in the triglyceride values. No consistent pattern of sex differences in the relative importance of genetic effects was apparent among the intraclass correlations.

In general, correlations for the monozygotic twins were higher than those for the dizygotic twins, indicating genetic influence. The importance of the influence of the rearing environment on levels of total cholesterol and apolipoprotein B was suggested by consistently higher correlations for the twins reared together than for those reared apart.

A-I suggest that differences in these concentrations were sex-related.

To examine further the linearity of age effects without arbitrary categorization of the sample into cohorts, hierarchical multiple regression analyses were also performed in which the lipid levels of one twin were predicted from the levels of the other twin, as well as by terms relating to zygosity and age

The extent to which monozygotic twins are more alike than dizygotic twins should therefore reflect genetic influences. In the classic twin method, the difference between intraclass correlations for monozygotic twins and those for dizygotic twins is doubled to estimate heritability. The remaining population variation can then be attributed to environmental factors.

none reported taking drugs prescribed specifically to lower lipid levels.

the sample consisted of 46 pairs of monozygotic twins reared apart, 67 monozygotic pairs reared together, 100 dizygotic pairs reared apart, and 89 dizygotic pairs reared together.

The SATSA subregistry was compiled in 1984 by contacting pairs of twins identified in the registry as having been reared apart, along with matched pairs of twins who had been reared together.

Although some epidemiologic studies have reported on lipid levels in the elderly,12–15 most genetic studies have focused on younger people

A common criticism of twin and family studies is that they may overestimate heritability by confounding genetic and environmental factors, in that relatives living together share environments as well as genes. A more ideal design is to combine a twin study with an adoption study

The genetic factors described to date have been mainly those involved in familial syndromes of lipid disorders, many of which are determined by single genes.

The effect of genetic factors on the serum levels of some but not all lipids appears to decrease with age