We question the premise that genetically modified (GM) marmosets are essential to Japan's brain-mapping project for studying disorders such as schizophrenia and depression (Nature 514, 151–152; 2014). It runs counter to efforts in fields such as toxicology to reduce or eliminate the use of non-human primates in research.

Our conclusions in the 2011 Review of Research Using Non-Human Primates (see go.nature.com/kgy27w), which included UK neuroscience research between 1997 and 2006, are relevant here. The review judged such research to be of high quality, yet few projects showed evidence of biomedical benefit — despite being completed many years before the review.

There was a tendency among researchers to make over-optimistic and unsubstantiated claims for their work and how it might improve the treatment of brain disorders, presumably to strengthen their case for funding. We concluded that “the size of the problem to which the science relates should not be accepted as sole justification for individual items of research” and that “health benefits should only be claimed when their potential is real”. In some instances, we judged that the case for using non-human primates — rather than other species, including humans — was not well made.

It could be argued that the unavailability of suitable GM animals at the time might account for the poor translation into health benefits. But the justification for using GM marmosets in research today needs to be much stronger, because of the substantial ethical and welfare issues at stake.

You quote US neuroscientist Terry Sejnowski, who proposes consideration of “the ethical issues that will inevitably arise up the road”. We contend that these should be considered before the journey starts.