Twin studies

Question 1

What are monozygotic (MZ) twins?

Answer 1

• When a single egg cell is fertilised by a single sperm cell, and the resulting zygote spilts into two seperate embryos very early in development.
• Also called identical twins.
• They share 100% of the same genetic material.

Question 2

What are dizygotic (DZ) twins?

Answer 2

• When two egg cells are fertilised by two different sperm cells in the same menstrual cycle.
• Also called fraternal twins.
• They share only 50% of the same genes of any two siblings.

Question 3

What is concordance rate?

Answer 3

• The extent at which both twins share the same characteristics (probability of them displaying the same behaviours).

Question 4

Generalisability of twin studies

Answer 4

LOW - Twin studies have to use twins as their sample, 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%).

Question 5

Reliability of twin studies

Answer 5

P - Twin studies are becoming increasingly reliable due to the rise in multiple births.
E - Because of fertility treatments and more women having children later in life, the number of twins being born is rising. This gives researchers access to larger samples, which means they can use more powerful statistical tests and it becomes easier to replicate studies.
E - This boosts the reliability and scientific credibility of twin research, as consistent findings across large, replicated studies increase confidence in the results.

Question 6

Application of twin studies

Answer 6

HIGH - 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.

Question 7

Validity of twin studies

Answer 7

P - Twin studies may suffer from reduced internal validity due to confounding variables.
E - Even though twins are raised in the same household, they may have different experiences, such as different friends, hobbies, or social roles. This means that although they share the same genotype (biological makeup), they may not share the same phenotype (expressed behaviours and experiences).
E - These environmental differences act as confounding variables, making it harder to determine whether similarities or differences in behaviour are due to genetics or individual environmental factors, thus reducing the internal validity of twin study conclusions.
CP - However, studying twins who are reared apart can help control for shared environmental factors, providing clearer evidence of genetic influence and improving the validity of the findings.