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.
Podíamos observar durante un tiempo prolongado a ciertas personas, personas que, por ejemplo, eran genéticamente vulnerables a la esquizofrenia y preguntar en qué contribuía la crianza, en qué la genética, y podíamos aislar otras variables realizando experimentos sobre las enfermedades mentales.