One explanation is that genetics can play a part in the development of Schizophrenia (SZ) and that the disorder can run in families.
It is suggested that, the closer the generic relatedness someone is to a person with SZ, the higher the likelihood they will get SZ too.

He found that if both parents suffer from SZ then there is a 46% chance that the child will as well.

If one parent suffers from SZ then there is a 16% chance the child will too.

If a grandparent has SZ then there is a 5% chance.
If the identical twin of the parent has SZ then there is a 17% chance.

This supports the explanation that SZ is genetic.



Criticism: 

However, family studies lack population validity. The findings cannot be generalised to the whole population. The samples within studies tend to be small and it is difficult to find families with two SZ parents and a SZ child.
It's also importance to note that genetics can't be the only explanation for SZ as the concordance rates would be 100%.
It can also be argued that environmental factors may explain the increase in chance of developing SZ between family members. This is because they tend to share the same environment and something in that environment could be influencing the symptoms.



Twin Study



Gottesman (1991)

Most twin studies show a higher concordance rate between Monozygotic (MZ - idenitcal) twins then Dizygotic (DZ - non-identical twins).

Gottesman's study was carried out on 40 twins and he found that the concordance rate in MZ twins was 48% and in DZ twins it was 17%.

Cardno (2002) also supported these findings and found a concordance rate of 26.5% for MZ twins and 0% for DZ twins.

These studies both suggest that SZ is genetically inherited because if there was no genetic inheritance, then there wouldn't be a difference between MZ and DZ twins.


Criticism:

However, because there is not 100% concordance rate it cannot be genes alone.

Once again, it could be due to the environment (Nurture) and we could argue that it's due to the fact that MZ tend to have greater environmental similarities. They also tended to be treated more alike than do DZ twins.

In addition to this though, the assumption that all MZ twins are raised the exact same way in not true. Therefore, we shouldn't make conclusions based around this view but just suggestions.

We should also be cautious when comparing and generalising the studies used. Some cultures may use different methods for diagnosing the disorder (reliability issue) and studies may use different methods for calculating concordance rates (validity issue).



Adoption Study:

Adoption studies are a more effective way of studying SZ as it allows the researcher to separate the environment from genes. This is because the children will have their parents genes but will have been brought up in a different environment to them.



Tienari (1991) 

In this study there were 155 adopted children, those of which biological mothers had SZ. They had a concordance rate of 10% compared to 1% in adopted children without SZ mothers.

This study shows evidence that genes do play a part in SZ as the concordance rate was higher in those adopted children with SZ parents, even though they did not share the same environment with their parents.


Criticism:

However, there is not 100% concordance rate and it's also still very low. This suggests there are still other factors that could play a part in the development of SZ and it is not genes alone.


Sum up:

Although there is a lot of evidence to support the fact that genes do play a part, looking at genes alone would be too simplistic and reductionist.

Instead, we would need to take a Diathesis-Stress Model approach which provides a better explanation for this disorder. This suggests that genetics are a vulnerability to the disorder and predispose a person to it. It's then something in the environment that may trigger the SZ to develop in a person.