Remember this scene from Trainspotting? Watching it makes you more prone to harsh moral judgements (Image: Miramax / Everett / Rex)

WILL these hands ne’er be clean?” asks Lady Macbeth, as she obsessively tries to wash away the guilt she feels for her role in the murder of King Duncan. Her feelings of self-disgust, we are led to believe, have manifested themselves as a sensation of physical dirtiness.

It is not only in the language of playwrights such as Shakespeare that complex emotions like guilt, grief or loneliness are compared to physical sensations. These metaphors crop up in everyday phrases, too, in many languages. In English, for example, we talk of being “left out in the cold” when we feel socially excluded, a sentiment echoed in the Japanese saying “one kind word can warm three winter months”.

At face value, these connections seem purely symbolic. In real life, loneliness doesn’t really send us shivering, and guilt doesn’t really make us feel dirty. Or do they? Recent research has found that these physical sensations can often accompany our emotions. It works the other way too – by provoking a feeling of disgust, a scene from the film Trainspotting shaped the way subjects in an experiment made moral judgements.

Many now believe that this reflects the way complex emotions arose in our evolutionary past. As our brain evolved to process more and more complex emotions, the theory goes, there was no need for new neural machinery: our emotions simply piggybacked onto the circuits that handle basic sensory perceptions. Here are some of the most striking experiments linking physical sensations with emotions and behaviour. …