Twin and adoption studies as research methods.

Twin and adoption studies as research methods.

Twin and adoption studies are used to study the influence of genes on behaviour. Twin and adoption studies relate to the nature-nurture debate which is about how far a characteristic comes from nature and how far it comes from out nurture. 

Identical twins are monozygotic (MZ) twins - this means that they have developed from one (mono) fertilised egg. Their DNA is 100% the same and are always the same-sex. If a characteristic is genetic, then MZ twins should both show that characteristic. Important points include: 

• MZ do not share the same environment, even in the womb, so they tend to develop differently, despite their DNA being the same.
• From the start, there are some small physical changes between the twins, such as fingerprints.
• Some characteristics that are genetic are triggered by the environment, so identical twins can become less identical over time, although, of course, their DNA does not change. 
• Some characteristics, such as IQ and personality, might become more alike as the twins age.

Non-identical twins are dizygotic (DZ), which means that they come from two fertilised eggs (di = two). Since they develop from different eggs, the DNA of DZ twins is only as similar as that of any siblings; it is not 100% the same, as it is in identical twins. This means that if a characteristic is genetic, DZ twins would be expected to share that characteristic to an extent but not as much as MZ twins would share it. 

Strengths & Weaknesses:

Strengths:

• Both MZ and DZ twin babies are born at the same time and share a similar environment, but MZ twins have exactly the same DNA as each other whereas DZ twins share about 50% of their DNA. So twin studies are the main way that the influence of genes on behaviour can be studied - there is no other way of having identical DNA.
• Both MZ twins and DZ twins share their environments and are treated as twins. For the most part, people treat twins as twins, whether they are MZ or DZ twns. So although there are differences between MZ and a DZ set of twins in terms of their shared genes, for either type of twin there should not be environmental differences between the twins themselves.

Weaknesses:

• MZ twins, being…