  However, we still face the question of how psychological deficits are related to other levels of explanation, and here Marr’s picture comes under real stress. Marr’s three levels are different representations of the same process. But in psychiatry different levels typically represent distinct causal processes. For example, the causes of many mental illnesses include a mix of genes and environmental factors, as in Meehl’s example, cited above, of genetics and bereavement conspiring to cause depression. The genes and environmental effects are different kinds of processes, not different levels of one process. If we were dealing with one process, describable in different ways, then we could anticipate a reductive account, in which higher-level variables are mapped on to lower-level ones. But even though it is hard enough to imagine a molecular or neurological reduction of a psychological construct like humiliation (which, we saw above, is depressogenic), it is even harder to imagine a reductive analysis of socio-cultural factors like unemployment or childhood sexual abuse. They have brain effects, but the brain effects vary across classes of individuals in ways that depend on other environmental and genetic contexts (see Kendler and Prescott 2006 for a comprehensive review.)