It is a beguiling offer — an investment that can produce a financial return and also a social/environmental benefit. Private benefit plus public benefit. It probably does happen sometimes but certainly not for every impact investment product. Investors must be on their guard.

The impact of something is the difference between the world in which that thing happens versus the world in which it does not. So assessing impact means considering what would have happened without it.

This is how to assess the impact of anything. For example what would have become of you if you had not been educated? What would have happened to Europe without the reparations demanded from Germany after World War I?

Establishing what would have happened otherwise, the counterfactual, is sometimes impossible as in the reparations example. In those cases we have to make reasonable conjectures based on everything else we know. Sometimes it is possible – though it may be complicated. This is a whole branch of social science.

Rigorous use of those scientific methods shows that at times, the impact of something is not what we imagine.

Read the full article about impact investments by Caroline Fiennes at Giving Evidence.