You won't need ALL the material on this page. You need to know about twin studies and adoption studies in general for the Biological Approach in Unit 1. They're also useful for discussion the biological explanation of mental disorders in Clinical Psychology (Unit 2) as well as biological explanations for criminality (Unit 2). A question might ask you to "use research evidence" in which case you must know findings from a study; a question could also ask you to describe or evaluate a study (but it won't identify a study by name).

WHY STUDY TWINS & ADOPTEES?

Twin studies and adoption studies are ways to investigate the effects of nature and nurture on behaviour.
  • Nature refers to biological causes for behaviour: this means the influence of genes that have been inherited from biological parents.
  • Nurture refers to the environmental causes of behaviour: this means the importance of upbringing, regardless of who the parents are.
It might help to remember that nUrture (with a 'U') is about Upbringing (with a 'U')
In general, twin studies investigate the role of nature (genes) and adoption studies investigate the role of nurture (upbringing).

Adoption studies and twin studies are always natural experiments. This is because the IV being investigated (whether you are biologically related to your family or what type of twin you are) is a naturally-occurring variable. You can't manipulate variables like these: it would be morally wrong to force children to be adopted to see what happens to them and it's scientifically impossible to cause twins to be born (though certain fertility treatment can increase the likelihood).

ADOPTION STUDIES

Adoption studies look at the impact of nurture on children who are raised by parents who are not their biological parents.

Because there is no biological connection between the parent and the child, if the child grows up to share the parents' traits (or the traits of their step-brothers or step-sisters who are biologically related to the parents), then these traits are probably produced by nurture.
It's hard to resist a Harry Potter reference here. Harry is adopted by the none-more-Muggle Dursleys, but he shows a magical talent they lack (as well as basic niceness). This suggests that magical ability is NOT down to nurture but rather down to nature - especially as we know that both of Harry's parents were wizards.
Adoption studies are made more valid if the researchers have information about the child's biological parent. If the child grows up with traits that resemble the adoptive parent more than the biological parent, this is stronger evidence that these traits are due to nurture.

For example, an adoption study might investigate intelligence. If an adopted child score just as highly for IQ as his or her adoptive parents or step-brothers or step-sisters, this suggests that intelligence is down to nurture. If the adopted child's biological parents scored very differently on IQ tests, this strengthens the conclusion that upbringing, not genetics, is responsible for intelligence.

On the other hand, if the adopted child's IQ scores do not resemble those of the adoptive family, this suggests nurture is not such a strong influence on intelligence. If the adopted child's IQ more closely resembles the biological parents, this further weakens the idea that nurture can influence intelligence.
Because you also look at twin studies, it's easy to get muddled up and think that adoption studies involve twins, one of whom has been adopted. That's not so. Adoption studies don't usually involve twins. These days, twins are almost always put up for adoption together and not separated.
Adoption studies are usually measured using a correlational technique: the researchers are looking for a correlation between the behaviour of the children and their parents.

EVALUATING ADOPTION STUDIES

Generalisability

Adoption studies have to use adopted children as their sample and this is a group that may not be very representative of other children. For one thing, these children have been separated from the biological parents, perhaps through tragic circumstances or because their biological parents "gave them up" due to difficult circumstances. Child Psychologists like John Bowlby argue that children are badly affected if they are separated from their mother during a critical period in infancy. If this happens to adopted children, it would make them even less representative of children in general.

Reliability

Adoptions are handled by charities or by state agencies and psychologists have to be "opportunistic" in recruiting children for adoption studies. There will be many extraneous variables at work:
  • Children may need to be adopted for different reasons (eg immigrant children or abandoned children compared to children whose parents have died)
  • The children may have spent some time as orphans and this early upbringing might affect them
  • They may have been fostered to other families before the current family which has adopted them and these earlier parents may have influenced the child

Psychologists try to get round these problems by conducting adoption studies with large samples, so that statistical analysis can "average out" the anomalies. However, this makes adoption studies difficult, time-consuming and expensive to carry out, especially because there aren't that many adopted children.

Application

Adoption studies can tell us whether good parenting can correct bad genes. Children whose biological parents were drug addicts, alcoholics or criminals may get a "second chance" if they are adopted by families with healthy lifestyles and adoption studies tell us how likely this is. These studies also tell us how ordinary families might raise children not to repeat the mistakes they made, like turning to crime or underachieving at school.

Validity

There are lots of confounding variables in adoption studies. Adoption agencies try to match children to adoptive families as similar as possible to their biological family (in terms of race, ethnicity, class, etc) and this makes it harder to tell if upbringing is at work.

The idea that your upbringing entirely shapes the person you grow up to be is environmental 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 adoption studies because, however an adopted child grows up, there will always be the possibility that he or she would have grown up that way anyway, however they were raised. After all, lots of children raised by their biological parents grow up to be completely different from their parents in many ways.
Ethics

Children cannot consent to be studied, but adoption studies usually proceed with the presumptive consent of the adoptive parents and (where possible) the biological parents too. The children's anonymity is preserved and this respects their privacy and dignity.

There is always a danger that adoption studies may create a rift in families, by drawing attention to the differences between the adopted children and their step-brothers or step-sisters, or drawing unfortunate comparisons to the children's biological parents. This can "create self doubt" which is a risk that the BPS warns about. Careful debriefing is needed in this sort of research.

Social responsibility is important in ethical research. Given that thousands of children are adopted every year (5330 in the UK in 2015), adoption studies benefit society by helping us understand how much (or how little) adoptive parents can do for the children they are raising.

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.

Reliability

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.

Validity

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 cause it.
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.

APPLYING & COMPARING TWIN & ADOPTION STUDIES
AO2 AND AO3 

The Edexcel course expects candidates to be able to describe, apply and evaluate twin and adoption studies, including two examples. Brendgen et al. (2005) is the Contemporary Biological Study and also a twin study.

Here are more:

COMPARING ADOPTION & TWIN STUDIES

Similarities

Adoption and twin studies are both natural experiments. They study the influence of genetics and environment on behaviour, but they do this by observing naturally-occurring variables rather than manipulating IVs in a lab setting. As such, they avoid a lot of the ethical problems with manipulating humans in lab settings.

Both of these methods focus on comparing people who share genetic material (MZ twins and biological parents and their children) with people who share less genetic material (DZ twins and adoptive parents and their adopted children) while environmental factors remain constant (both types of twins share the same home; adopted children live with their adoptive parents). This highlights whether or not genes (nature) influence behaviour more than environment (nurture).

Differences

Twin studies are much better at studying the effects of nature (genes). This is because MZ twins share 100% of their genes, whereas adopted children do not share 100% of their genes with either of their biological parents. High concordance between MZ twins tells us more about genetic influences than high concordance between children and their biological parents.

On the other hand, adoption studies are much better at studying the effects of nurture (upbringing). This is because we cannot be sure how much or how little twins share the same environmental influences (or phenotype). However, we can be sure that adoptive parents provide almost all of the upbringing for adopted children, but the biological parents contribute little or nothing.

Adoption studies don't face the problem of assigning zygocity to twins, which can be unreliable. However, modern DNA testing has greatly increased the reliability of this.

Twin studies don't face the problem of tracking down biological parents and persuading them to take part in the study. Because of migration and ease of travel in the 21st century, this problem may be greater now than it used to be.

EXEMPLAR ESSAY
How to write a 8-mark answer

Evaluate the use of twin studies in psychology. (8 marks)
  • A 8-mark “evaluate” question awards 4 marks for describing the twin study method (AO1) and 4 marks for evaluating it (AO3). You need a conclusion to get a mark in the top band (7-8 marks). "Twin studies" turn up in the Specification for the Biological Approach but could also be used in a question on nature/nurture as part of Issues & Debates.

Twin studies avoid the ethical problem of manipulating human genes in a laboratory.  They do this by investigating genetic differences as a naturally-occurring variable, because MZ twins naturally share a genotype with their twin.

However, twins are hard to generalise from because they are rare. Only 1.5% of UK births are twins and only a third of them are MZ twins.

The results from twin studies might be invalid if there is a different explanation for the twins' behaviour other than genes. Twins are treated differently from other children by outsiders and may be mistaken for each other, putting pressure on them to copy each other's behaviour.

Another validity problem is that it's hard to identify MZ twins without DNA tests. DZ twins can look similar and have the same blood group and similar fingerprints, which are commonly used tests.

In conclusion twin studies have improved because psychology has progressed over time. Twin studies now use DNA testing, but even Brendgen et al. (2005) relies on identifying zygocity by appearance.
  • Notice that for a 8-mark answer you don’t have to include everything about twin studies. I haven’t mentioned ethical problems or comparisons with adoption studies. But it is a balanced answer - half description, half evaluation.
  • Notice that I introduce the evaluation point first then follow it up with description/knowledge; I try to link the conclusion to an Issue/Debate (but this isn't compulsory).