A new study in PLOS one (freely available) examined the brains of vegans, vegetarians and omnivores using functional MRI. Functional MRI machines enable researchers to see which areas of the brain are being used during some activity. A particular area will be active for almost everybody when they are talking ... it's called Broca's area. If it is damaged, then most people can't speak. But if the damage is slow then the brain may move the function to another area. Brains are very clever!  There are also areas which are strongly associated with empathy. Show vegetarians and vegans images of people being hurt and these areas light up ... and light up more strongly than in omnivores. Show vegetarians and vegans images of animals being hurt and there is a whole raft of differences between vegetarians, vegans on the one hand and omnivores.

In psychological tests run during the research, there was a significant difference between empathy scores in vegans and vegetarians as a group and omnivores. These supported the fMRI results and showed omnivores have lower empathy scores. There were also differences between vegans and vegetarians in the fMRI results, but its not entirely clear how to interpret these.