For a concrete example of change in multilevel systems, consider the most effective treatment for
cases of serious depression. The available evidence suggests that the best way to improve the mood
of depressed people is a combination of cognitive therapy and antidepressant medication. This
combination intervenes at all four relevant levels: social, psychological, neural, and molecular. A
cognitive therapist assists patients at the psychological level by helping them to identify and
overcome negative beliefs, goals, and emotions. The therapist often also assists the patient in
improving personal and work relationships, so cognitive therapy intervenes at the social level as
well. Antidepressants such as Prozac and Wellbutrin affect levels of neurotransmitters including
serotonin and dopamine, so they operate at the molecular level, but they also change the firing rates of
neurons, as well as the generation of new neurons in the hippocampus. Hence it seems that the
combination of cognitive theory and antidepressant medication is beneficial owing to their synergistic
intervention at all four relevant levels of mechanisms. More effective interventions will become
possible through greater knowledge about the social, psychological, neural, and molecular
mechanisms underlying depression, including their interconnections described in conjectures 2-4.
Many more examples of changes in complex systems are needed to evaluate the plausibility of my
conjectures about system change. We desperately need to develop further evidence-based theories
about how to change psychological, political, and social systems in ways that can address the
daunting list of problems that humans now face. The account of multilevel explanations that I
defended in chapter 5 should pave the way for multilevel interventions that avoid simplistic models
of causality.
Figure 10.2 displays four commonly advocated views of such relations. The most familiar is (A),
the classical reductionist view that changes at lower levels cause changes at higher levels. On this
view, causality runs upward, and so should explanation: social changes are explained as the result of
psychological changes, which are the result of neural changes, all the way down to subatomic
changes. In the social sciences, some writers go far in the other direction, suggesting that the social
level is the key source of causality, as in (B). On this view, causality and explanation run only
downward, from the social to the psychological, and everything is a social construction; the neural
and molecular levels are largely ignored.