We could look across time at the same people -- people, for example, who were genetically vulnerable to schizophrenia -- and ask what the contribution of mothering, of genetics are, and we could isolate third variables by doing experiments on the mental illnesses.
Da bismo iste ljude mogli pratiti tijekom vremena - ljude koji su, primjerice, genetski skloni shizofreniji - i ispitivati utjecaj odgoja, ili genetskih faktora, a mogli bismo i izdvojiti treće varijable kroz pokuse s  mentalnim bolestima.