We will readily describe a person's demeanour as "warm" or "cold" but this link between temperature and personality is more than just a metaphorical one. A new study shows that warming a person's fingertips can also bring out the warmth in their social relationships, pushing them to judge others more positively and promoting their charitable side.

Lawrence Williams at the University of Colorado and John Bargh from Yale University managed to influence the behaviour of a group of 41 volunteers without them knowing it by giving them something warm to hold. When the recruits arrived at the psychology building, a colleague (who wasn't aware of the experiment's goals) escorted them to the laboratory and asked them to hold a cup of coffee for her along the way. Once in the lab, they had to read a description of a stranger and rate them on 10 different personality traits.

The cups of coffee were the key element. Half were hot and half were iced, and the volunteers' brief contact with the cups was enough to sway their later impressions. The recruits whose hands were heated by their cups rated the stranger as having a warmer personality than those who held the cold cups. On average, they gave him a score of 4.7 on a scale of 1 to 7, while the cool-handed people gave average scores of 4.3.

The difference was small but statistically significant, and it wasn't just a symptom of a generally improved mood brought on by a steaming cup of java. After all, Williams and Bargh also found that the temperature of the coffee cups had no bearing on how the recruits judged the stranger along personality traits unrelated to warmth of personality. There was, however, a small chance that the accomplice was subtly influencing the volunteers' behaviour since she too had clutched the cups of coffee.

With that in mind, Williams and Bargh gave another group of 53 people an envelope of instructions, asking them to hold a hot or cold therapeutic pad under the premise of evaluating it. Later, when the unknowing volunteers were offered a small reward for their trouble - a bottle of Snapple or a dollar voucher for the local ice-cream parlour - those who touched the hot pads were more likely to give it to a friend than to keep it for themselves. Three-quarters of them chose this charitable option, compared to just 54% of the recruits who held the cold pad.

Together, the coffee and pad experiments show that sensations of can both affect a person's judgment of, and influence their actions towards, other people. In both cases, the recruits made their choices freely and were unaware that they had been subtly manipulated.

The small size of the effect might make it easy to dismiss the result as a kooky psychological curio; after all, in a real-life situation, surely other factors like sense of humour or personal hygiene would have a greater impact on first impressions? Perhaps so, but psychological research has shown that the warm-cold dimension is incredibly important to us when we meet other people. Our first impressions of strangers tend to be based primarily on how warm we think they are and, to a lesser extent, how competent. We make warm-cold judgments quickly and automatically, and they provide us with a quick overview of other important personality traits including friendliness, trustworthiness and helpfulness.

It seems that our minds cope with abstract psychological concepts by rooting them in metaphors based on solid physical experiences Many psychologists claim that this association stems from early infancy when the close embraces of our parents provide us with both body heat and security, forever linking physical and personal warmth in our minds. Later on in life, the heat of a cup of coffee may recall these earlier warm experiences and the feelings of trust and comfort that are associated with them. It works the other way round too - I've blogged before about another study which showed that making someone feel socially excluded can literally make them feel colder.

Neuroscience studies also support the idea of a close link between abstract concepts and physical sensations; some have shown that a part of the brain called the insula is involved in processing feelings of warmth in both their physical and psychological incarnations. Neurons in the right half of the insula are more active the warmer an object is to the touch, and other insula neurons are specialised to deal with feelings on trust and empathy.

Reference: L. E. Williams, J. A. Bargh (2008). Experiencing Physical Warmth Promotes Interpersonal Warmth Science, 322 (5901), 606-607 DOI: 10.1126/science.1162548