A third theory as to the link between smoking and schizophrenia suggests that there are common genetic risk factors for nicotine dependence and schizophrenia. However, in a Swedish study comparing pairs of identical twins, full sisters, half sisters, and female cousins, the research team found that there was no significant decline in hazard ratio for developing schizophrenia across relatives with less genetic relationship. When looking at the identical twin pairs, it was found that the twin who was a heavy tobacco smoker was 1.7 times more likely to develop psychosis compared to her non-smoking twin. This therefore suggests there is not strong evidence for the “common genetic risk factors” theory.