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Addiction » Kaij's Twin Study of Alcohol Abuse

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.2.4.7

The aim, method, results and conclusion of Kaij's twin study of alcohol abuse
How the results support the genetic explanation of addiction
Strengths and weaknesses of the study
How to write about a named study in the exam

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Why study twins?

In the last lesson you saw that identical twins share all of their genes, non-identical twins share about half, and that concordance tells us how alike twins are. A twin study is a clever way to test whether genes affect addiction. This lesson looks at one real example: Kaij's twin study of alcohol abuse.

Key terms:

Alcohol abuse: drinking alcohol in a harmful way that causes problems for the person, for example with their health, work or family.
Registered: having your name on an official list. In this study, men were on a list because of their alcohol problems.

The study

🎯 Aim

To find out whether genes play a part in alcohol abuse. Kaij wanted to see if identical twins were more alike in their drinking problems than non-identical twins.

📝 Method

The study took place in Sweden. Kaij used male twin pairs where at least one twin was registered for alcohol problems. The twins were classed as identical or non-identical. They were then interviewed about their drinking.

📊 Results

Identical twins were much more likely than non-identical twins to both abuse alcohol. The concordance was roughly 54% for identical twins and roughly 28% for non-identical twins.

💡 Conclusion

Genes play a part in alcohol abuse. Identical twins share more genes, and they were more alike, so inherited genes may make some people more vulnerable.

Learn this

Roughly 54% concordance in identical twins against roughly 28% in non-identical twins. Always say "roughly" or "about", and remember that the study was of male twins in Sweden.

Making sense of the results

Think about what the numbers mean. If one identical twin had an alcohol problem, in roughly half of the pairs the other twin had one too. For non-identical twins it was only about a quarter.

The main difference this study looks at between the two types of twin is how many genes they share.
Identical twins were about twice as concordant as non-identical twins.
This supports the genetic explanation of addiction (the idea of genetic vulnerability).

But 54% is not 100%. In many identical pairs, one twin had a problem and the other did not. Twins with the same genes could not differ like this if genes were the whole story, so something else must matter too.

Worked example: using the results

A question says: "What do Kaij's results suggest about alcohol abuse?"

Reasoning:

Identical twins: roughly 54% both abused alcohol.
Non-identical twins: roughly 28% both abused alcohol.
Identical twins share more genes and were more alike, so genes play a part.
The answer is: "Identical twins were more concordant than non-identical twins, which suggests that genes play a part in alcohol abuse."

Evaluating the study

✅ Strengths

It compares identical and non-identical twins, who differ in how many genes they share.
The twins came from an official register, so there was a clear group of people with known alcohol problems.
The result is clear: identical twins were much more concordant.

❌ Weaknesses

Identical twins may also share more of their environment. They may be treated more alike, so it is not certain the difference is down to genes.
Concordance was not 100%, so genes are not the only cause.
Only males were studied, so the results may not apply to women.
It is an old study, and attitudes to drinking and ways of spotting problems have changed.
Twin type was judged without DNA tests, so some twins may have been put in the wrong group.

Writing about a study in the exam

Examiners like an answer that follows a clear order. Use this one for any named study: aim, method, results, conclusion, then a strength and a weakness. Put the findings in your own words and link them back to the question. For Kaij, always link the results to genes and to alcohol abuse.

Common mistakes

Saying the study proves genes cause alcohol abuse. It suggests genes play a part, but concordance was not 100%.
Mixing up the figures. Identical twins are about 54% and non-identical twins about 28%.
Forgetting that the twins were male and from Sweden.
Saying identical twins have "different genes". They share all of their genes.
Giving a vague weakness such as "it was not good". Name the problem, for example identical twins share more environment too.

Exam-style question

(a) Describe the method used in Kaij's twin study of alcohol abuse. [2 marks]

(b) Kaij found that identical twins were more concordant than non-identical twins. Explain what this suggests about alcohol abuse. [2 marks]

Model answer

(a) Male twin pairs in Sweden were used, where at least one twin was registered for alcohol problems (1). The twins were classed as identical or non-identical and interviewed about their drinking (1).

(b) Identical twins share all of their genes, but non-identical twins share only about half (1). Identical twins were more alike in their alcohol abuse, so this suggests genes play a part in it (1).

(c) Identical twins may share more of their environment as well as their genes (1), so it is not certain that the higher concordance was caused by genes alone (1). Other valid weaknesses are accepted, for example that only males were studied.

Exam tip

For a weakness, make the point and then say why it matters. In (c), do not stop at "only males". Add that the results may not apply to women.

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