This sort of claim about causality has consistently proved controversial. Oliver James recently ignited firm criticism from behaviour geneticists when he baldly denied the role of genetics in mental health problems. I am with the behaviour geneticists in that dispute; James' dogmatic environmentalism rests on a wilful misunderstanding of scientific findings, and on some very shaky arguments.
Environmentally inclined clinical psychologists often want to push back against a view that says most of the cause of mental health problems lies in our genes. There is a fact of the matter about this, and it does suggests a powerful role for pre-disposition. If we want to find someone who meets criteria for schizophrenia our best bet is to find someone who has an identical twin with the disorder. Nothing else raises the risk so far (from its baseline of around 1% to 28%*). Because of this, many researchers now hold that bio-genetic vulnerabilities do the bulk of the causal work in psychosis (leading some psychologists to complain that environmental factors are marginalised by being reduced to the status of "trigger").
But even so, the claim that we underplay the environment's role as a cause may be warranted. Causality is complex and we assign different weights to different causal stories depending on what we intend to use them for. A criminal court, for example, may apply a "but for" test, asking whether the events under examination would have happened but for the actions of a defendant. This doesn't necessarily show us the full causal picture as it doesn't answer questions about why the defendant behaved as they did (indeed liberally inclined thinkers tend to feel that the criminal justice system focuses too much on individual responsibility and not enough on societal causal factors when punishing people), but it works tolerably well for assigning a certain sort of criminal responsibility.
Bringing environmental factors further into the foreground may serve a valuable purpose in the mental health debate. Consider this passage from Peter Zachar's book A Metaphysics of Psychopathology:
Zachar brings out the element of choice we have in identifying causes. Exactly what we choose to call a cause depends in part on what aspects of the whole situation we consider "background conditions". He does not imply that the choice is limitless (he is not a relativist about causes), but he does suggest that where you turn your investigative attention may legitimately be a function of your interests; a function of what aspects of the total situation you feel to be most relevant.
What motivates psychologists who see trauma and "life events" as significant in causing mental distress is a refusal to see various forms of adversity as a "background condition". Sure, genetics plays an important role, these researchers suggest, but the public health implications of that fact are not immediately clear. Meanwhile, the public health implications of an aetiological role for traumatic life events are obvious; we should aim to stop people being exposed to them. As Peter Kinderman says in the article I linked to at the top, "when unemployment rates go up in a particular locality you get a measurable number of suicides".
If asked, I am sure Kinderman would deny that a change in economic circumstances is the whole causal story in any given suicide. Likely a host of factors (personality variables, social support network and so forth) combine to create something like more or less "resilience" in people. But unless you can intervene to improve that resilience, it makes sense to push it some way into the background and focus on things you feel you can change. If you do this, life circumstances and political events start to look more like "difference makers", even if we can still have a debate about what constitutes a cause.
* UPDATE: I originally cited the figure 48% here, reflecting the commonly quoted probandwise concordance rate for schizophrenia in identical twins. 28% reflects a lower estimate of concordance, based on a pairwise concordance rate. There is some controversy over which rate to cite, and as this post was an argument for greater focus on environmental factors, I did not want to lay myself open to the charge of minimizing the genetic contribution. However, it was suggested to me that the probandwise rate is an inflation of the true concordance rate, and for the time being I'm inclined to agree. Nonetheless, there are good arguments for using the probandwise concordance rate, and when I have better understood the issue, I will try to write a post outlining them.