Another important method for autism research is the twin study. This involves comparing identical twins, who originate from a single fertilised egg and have identical genes, with non-identical twins, who come from two different eggs and who, on average, share 50% of their genes – just as siblings do. This type of study regularly finds that when one member of an identical twin pair has autism, the second twin is more likely to have some form of autism than when the twins are non-identical, providing evidence of a strong genetic influence. Studies of both twins and wider family members have shown that autism is highly heritable, meaning that genetic factors play a major role in determining whether individuals within a population will develop autism. However, this heritability is very complex. For instance, while some genetic variations linked to autism are ones that offspring inherit from their parents, others may arise afresh, affecting just one individual or a pair of identical twins within a family. Some non-genetic factors may also contribute to causing autism. These complex factors are further discussed in section 5.3.