Twin studies allow researchers to assess hereditary influences, in part, because

TWIN STUDIES

In order to understand twin studies, you need to understand the two types of twins:

• Monozygotic (MZ) twins were conceived in a single egg, which later split. These twins share the same genes. This means they MUST be the same sex. They are also called identical twins - and they usually look identical, although development in the womb and later health and diet can make them look different.
• Dizygotic (DZ) twins were conceived when two or more eggs were fertilised at the same time. These twins share the same amount of genes as any brother or sister, up to 50% in the case of same-sex DZ twins, but DZ twins may be of different sex. They are also called fraternal twins and they don't have to look identical - although they may be very hard to tell apart.
  

Sometimes you hear about triplets and quadruplets - or even octuplets where a set of 8 twins are born. Usually multiple births are triggered by fertility treatment. Typically, some of these twins are DZ (because there was more than one egg which was fertilised) and some MZ (because some of the fertilised eggs split and maybe even split again).

Twins are rare. The chance of a mother having twins in the UK is

1.5%.  Only a third of twins are MZ; DZ twins are more common.

• The odds of having twins increase if you're an older mother (over 35) or using fertility treatment or Nigerian (in Nigeria, 5% of births are twins and no one knows why).
• The odds decrease if you're a younger mother (under 25) or Asian, especially Japanese (0.3% apparently).

Identifying zygocity (whether MZ or DZ) is not as easy as you would think. Obviously, if the twins are different sex, then they must be DZ; if they have different blood groups, they must be DZ too. If they look very different (different eye colour, different hair colour) then they are probably DZ. But a similar-looking pair of same-sex twins with the same blood group may be either MZ or DZ. Fingerprints may help, because MZ twins have similar fingerprint patterns, but not identical ones, so DZ twins could have similar fingerprints too.

In the past, there was no 100% way of identifying MZ twins; there was a big "grey area" for similar-looking same-sex twins. However, two inventions since the 1970s have changed that.

• Ultrasound scanning may reveal twins sharing a placenta or the same membrane, which massively increases the likelihood they are MZ.
• Genetic testing can be carried out to reveal with much greater certainty whether twins are MZ

Twin studies from before the mid-1980s when ultrasounds became commonplace suffer from the problem of muddling MZ and DZ twins up.

The important bit...

Twin studies involve comparing MZ twins with DZ twins.

It's worth letting that sink in. Twin studies aren't about comparing twins with normal brothers and sisters. They're about comparing the two different types of twins.

Because MZ twins share 100% of their genes and DZ twins only share 50% (at most) of their genes, studying the behaviour of twins reveals a lot about the influence of nature (genetics).

• If the MZ twins and the DZ twins behave the same way, it suggests nature does not play a large role in this behaviour, because they twins share the same environment (nurture) which encourages them to be the same
• If the MZ and DZ twins behave differently, it suggests nature is at work, because the shared environment is not producing the same behaviour, so genetic differences could explain this

A famous example would be the Genain quadruplets. Born in 1930, these identical (MZ) German sisters all developed schizophrenia, which suggests a "schizophrenic gene" is at work. The mother and father also had family histories of mental illness, which adds to the credibility of the theory that schizophrenia is at least partly due to nature.

CONCORDANCE RATES

Twin studies are measured using a statistic called a concordance rate. Concordance means "agreement":

• If one twin shows a behaviour and the other twin does too, this is concordance
  
• If one twin shows the behaviour but the other doesn't, this is discordance
  

The concordance rate is usually expressed as a percentage (100% concordance means all the twins shared the behaviour in common, 0% means none of them did). In a twin study, the concordance rate of the MZ twins is compared to that of the DZ twins:

EVALUATING TWIN STUDIES

Generalisability

Twin studies have to use twins as their sample (dur!) and this is a group that may not be very

representative of other children. For one thing, twins are rare (1.5% of UK births) and MZ twins are rarer still (0.5%). We still don't fully understand the processes that lead to twinning, especially MZ twinning, and there may be other special or unusual features of MZ twins beside their genetic similarities.

Twins may have unusual lives in other ways too. They attract attention and often get treated the same way even if they have different personalities; they may be mistaken for one another. This may cause them to start behaving more like each other than other brothers or sisters do.

Because of fertility treatment and women having babies later in life, the number of multiple births is increasing. This increases the number of twins available for study, making it possible to conduct research with large samples, analyse data with more powerful statistical tests and making it easier for other psychologists to

replicate studies.

However, the identification of

zygocity (MZ and DZ) is not perfect. For many studies from before the 1980s it is very unreliable indeed. For example, Gottesman & Shields (1966) had to use blood tests and fingerprint comparisons; this means DZ twins could be wrongly assigned to the MZ condition. Even modern studies like Brendgen et al. (2005)only used DNA testing on half of the twins studied and the rest were allocated to MZ or DZ based on physical resemblance. However, the DNA testing backed up the allocation to MZ/DZ 94% of the time; Brendgen considered this to be reliable.

Application

Twin studies can tell us whether important behaviours are

heritable - which means they are passed down genetically from your parents rather than learned from your environment. This is important for treating disorders like schizophrenia as well as alerting parents to the risks of children growing up with these problems. If "at-risk" children can be identified, then it might be possible to prevent problems occurring with correct parenting, education or healthcare. For example, schizophrenia seems to have a genetic basis, but certain life events must "trigger" the genes; people with a genetic predisposition towards schizophrenia should not abuse drugs or alcohol because this can trigger the illness.

Twins are an example of a

naturally-occurring variable being perfectly changed for study: MZ twins share 100% of their genes, DZ twins share about 50%. Because researchers are not manipulating this variable themselves, it reduces the risk of researcher bias.

There are

confounding variables in twin studies. Just because twins share the same home and parents, it doesn't mean their upbringing has been exactly the same. They may have different friends, interests and relationships and these differences may grow more pronounced as the twins grow older. THe psychological terms for this are that the twins may share the same genotype (biological similarity) but they may not share the same phenotype (social similarity).

The idea that your genetics entirely shapes the person you grow up to be is

biological determinism. However, most psychologists today are not determinists; they think that people are influenced by a mixture of environmental and genetic influences. This limits the usefulness of twin studies because they rarely show a concordance rate of anything like 100%. The best they can show is that concordance is slightly higher in MZ twins than DZ, suggesting that genetics influences this behaviour, but doesn't entirely causeit.

Ethics

Recognising the importance of twins for research into nature and nurture, many states keep records of twin births and invite families to join twin study programmes. This provides a panel of families who have

consented for their children to be studied. Brendgen et al. (2005) recruited twins from a panel such as this in Quebec, Canada.

Other researchers advertise for adult twins to

volunteer to be in twin studies. Gottesman & Shields (1966)appealed for twins on an old hospital record to come forward and be studied. There are no ethical problem with informed adult volunteers (although Gottesman & Shields were studying patients with schizophrenia, which raises separate ethical issues).

There is always the

risk that twin studies may draw attention to twins and make them feel unusual (or more unusual than they already are). There is a particular risk of children being made to feel "weird" or "different" and the BPS Code of Conduct warns against research that "damages self-confidence". However, psychologists often argue that there is so much benefit from twin studies that they have a social responsibility to study twins. Moreover, most twin studies only subject twins to the sort of attention and curiosity they already receive anyway as part of ordinary life.