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.
我们可以在很长的时间跨度里观察一些人 比如在基因上对精神分裂症易感的人群 我们想知道后天照顾和先天基因在这其中扮演的角色 我们可以通过对这个精神疾病的实验 分离出导致精神疾病的变量