 A popular method in determining how genes and environment ( " nature and nurture " ) contribute to a phenotype is by studying identical and fraternal twins or siblings of multiple births . Because identical siblings come from the same zygote , they are genetically the same . Fraternal twins are as genetically different from one another as normal siblings . By analyzing statistics on how often a twin of a set has a certain disorder compared to other sets of twins , scientists can determine whether that disorder is caused by genetic or environmental factors ( i.e. whether it has ' nature ' or ' nurture ' causes ) . One famous example is the multiple birth study of the <unk> quadruplets , who were identical quadruplets all diagnosed with schizophrenia . 
