FOR some people in this world, the glass always seems to be half-full. For others it is half-empty. But how someone comes to have a sunny disposition in the first place is an interesting question.

It has been known for a long time that optimists see the world selectively, mentally processing positive things while ignoring negative ones, and that this outlook helps determine their health and well-being. In recent years, it has also become clear that carriers of a particular version of a particular gene are at higher risk than others of depression and attempted suicide when they face traumatic events. The gene in question lies in a region of the genome that promotes the activity of a second gene, which encodes a protein called the serotonin transporter. Serotonin is a messenger molecule that carries signals between nerve cells, and it is known to modulate many aspects of human behaviour, although the details are complex and controversial. The transporter protein recycles serotonin back into the cell that produced it, making it available for reuse, but also reducing the amount in the junctions between cells and thus, it is presumed, the strength of the signal.

It has looked increasingly likely, therefore, that genes—particularly those connected with serotonin—have a role to play in shaping a person's outlook. So Elaine Fox and her colleagues at the University of Essex, in Britain, wondered whether genes play a part in the selective attention to positive or negative material, with consequent effects on outlook.

To find out, they took samples of DNA from about 100 people and then subjected these people to what is known as the dot-probe paradigm test to see how they reacted to different stimuli. In this test participants are briefly shown photographs that may be positive, negative or neutral in tone. They then have to press a keypad to indicate when a dot has appeared on the screen. It has been found by experience that the more distracting an image is, the longer a person takes to respond when the dot appears. That allowed Dr Fox and her team to discover how distracting particular people found particular images.

In a paper just published in the Proceedings of the Royal Society B they report that, sure enough, gene-related variation caused a bias in attention towards positive and negative material. Some people had two “long” versions of the promoter gene (one inherited from each parent), a combination that reduces the amount of serotonin in the junctions between nerve cells. These individuals were biased towards positive images and away from negative ones. By contrast, those who had either a long and a short version of the gene, or two short versions (and thus, presumably, more serotonin in the junctions), did not have such protective biases. In other words, the optimists really did see the world differently.

Rose-tinted spectacles may be good for one's health, as these results fit in with wider ideas about how a tendency to look on the bright side of life is part of being resilient to stress. Those with short variants of this gene are expected to have an increased susceptibility to mood disorders following such stress. It is not all good news, though, for optimists. Because these results suggest that a person's attitude to life is inherited, they serve as a stark warning to all buoyant optimists that trying to cheer the rest of the world up with nothing more than a smile and an effortlessly sunny disposition is doomed to failure.