As lead author on the research paper discussed in this article, I'd like to respond to this comment.
Its worth noting that this difficulty applies to all psychiatric research. The heritability estimates calculated by geneticists are just fancy correlations, for example, but everyone tends to forget this. Of course, its extremely difficult to prove causality but, in our paper, we judge our findings against the widely accepted criteria proposed by Hill (1965). These are:
(i) Strength of association - we show the association is strong (about as strong as the association between smoking and lung cancer)
(ii) Consistency in the data - we show that different research designs have yielded strikingly similar effects
(iii) Specificity - in a separate paper in the same journal (Bentall, R. P., Wickham, S., Shevlin, M., & Varese, F. Do specific early life adversities lead to specific symptoms of psychosis? A study from the 2007 The Adult Psychiatric Morbidity Survey. Schizophrenia Bulletin, available online) we show that specific types of trauma are associated with specific symptoms of psychosis (sexual abuse is specifically associated with hallucinations, for example)
(iv) Temporal relationship - childhood trauma precedes the onset of psychosis.
(v) Dose-response relationships - we show that the more severe the abuse, the greater the risk of psychosis.
(vi) Plausibility in terms of mechanisms - we have carried out research on plausible mechanisms, published elsewhere (Varese, F., Barkus, E., & Bentall, R. P. (2011). Dissociation mediates the relationship between childhood trauma and hallucination-proneness. Psychological Medicine, 42, 1025-1036.)
(vii) Reversibility - when rates of smoking go down, cases of lung cancer decline also. Sadly, we are in no position to test the reversibility of the effects we have observed.
(viii) Consideration of alternative explanations - we are not aware of any plausible alternative explanations.
The skepticism within psychiatry about the role of early adversity in psychosis is very reminiscent of the skepticism that met Hill when he suggested that smoking leads to cancer (which is why he proposed his criteria). We think we have strong evidence of causality (the dose-response effects are particularly difficult to account for in any other way) but its true we don't have an affidavit from God.