AS THOSE who follow American politics know all too well, Republicans and Democrats sometimes act as if they belonged to different species. This, it seems, is more than mere metaphor. A growing body of research is shedding light on ways in which partisans are indeed biologically distinct. According to one famous study, conservatives are not just more god-fearing than liberals (as Americans call left-leaning folk). They are more fearful in general, making them more receptive to threatening aspects of the environment. Hence, the argument goes, their penchant for tougher policing, harsher sentencing, stronger armed forces and other Republican shibboleths.

However, this observation does not by itself explain liberals' preoccupation with progressive policies which often aim to make people's lives more pleasant, as opposed to less unpleasant. Michael Dodd, of the University of Nebraska, wondered whether this is because they are drawn more strongly than conservatives are to the bright side of life. As he and his colleagues report in the Philosophical Transactions of the Royal Society, this does in fact appear to be the case.

To arrive at his conclusion, Dr Dodd tested how 46 self-professed right- and left-leaning Nebraskans react to a series of 33 images. Some were associated with negative feelings: a spider on a man's face (fear), an open wound with maggots (disgust) and a man being beaten up by a mob (anger). Others—a smiling child, a bowl of fruit or a cute rabbit—were picked to evoke a warm and fuzzy sensation (positive emotions fall less readily into distinct categories).

The level of arousal was measured by tracking changes in how the participants' skin conducts a tiny current. The nervous system reacts to emotionally salient stimuli by spurring eccrine glands to release moisture. Since more moisture makes skin a better conductor, an uptick in conductivity reflects heightened arousal (a phenomenon polygraphers exploit to help detect whether someone is lying). The results confirmed that nasty pictures aroused Republicans more than pleasant ones did. And, as Dr Dodd expected, the opposite was true for Democrats. In both cases, the more partisan the participant, the more pronounced the respective predilection.

But would Democrats also home in on nice things more readily than Republicans when presented with a mix of pleasant and unpleasant stuff? In a follow-up study, Dr Dodd recruited 76 undergraduates of different political persuasions and employed eye-tracking kit to follow their gaze as they were presented with collages of unpleasant and pleasant pictures. This time it turned out that both sides of the political divide fixated on nasty images more quickly than on nice ones. Both groups also dwelt on them for longer. (This makes evolutionary sense: nasty things can do harm, and so merit more attention than those which are pleasant, and mostly harmless.) But the effect was much subtler among liberals than among conservatives. For instance, they looked at the nasty and nice pictures for just under 2 seconds and just over 1.5 seconds, respectively. The figures for conservatives were about 2.8 seconds and 1.2 seconds.

These findings do not answer the question whether conservativism and liberalism are hard-wired. But, as Dr Dodd notes, this may not matter. Once the distinct physiologies are in place, be they acquired or innate, they are hard to dislodge—and no doubt spur some on the right to dismiss those on the left as ignorant hedonists just as some on the left slag those on the right off as obsessive fearmongers. Moderates will find none of this reassuring. Biology is, after all, more formidable an obstacle to compromise even than ideology.