Placebo and social observational learning
One of the greatest enigma in health is the human response to placebo. Placebo itself is an inert substance or treatment that has no effect – yet humans can respond with physiological changes as if the substance was active. For years some unscrupulous medical practitioners have used this response in people experiencing chronic pain as evidence that their pain is ‘all in the head’, or that their problem is ‘psychosomatic’, whereas other even less scrupulous snake oil merchants have used this as a way to sell things like crystals, colour therapy and even coloured lotions for the ‘healing’ of pain and other assorted symptoms.

Colloca and Benedetti are two of the most respected researchers into the phenomenon of human response to placebo. They have used a wide range of experimental methodologies to investigate placebo, and this one is yet another to add to their extensive repertoire.

In this study, hoping to investigate the effect of learning through observing someone experience placebo analgesia as compared with first-hand experience and verbal suggestion alone. The premise is that some placebo analgesia is influenced by expectancy, some by conditioning, some by reinforcement – and in this experiment, by social observational learning.

Social observational learning is where an individual watches another person and learns through ‘vicarious learning’.
In this experiment, the participants were asked to sit beside a person who had been trained to simulate the
experimental session. This person ‘always rated as painful the stimuli paired to red light and as non-painful the stimuli paired to green light. In this way, he simulated an analgesic benefit following the presentation of the green light.’ After observing this, the participant underwent his or her own experimental session.

The other two conditions were – one in which the person was conditioned using an electric shock paired with the red light, and were told a ‘sub-threshold’ electric shock would be delivered paired with a green light. An electric shock was never paired with the green light at all, leading to a conditioned response where the green light produced an analgesic effect. As the authors state: ‘It is important to stress that the stimulus intensity was surreptitiously reduced, so that the subjects believed that the green light anticipated analgesic effects’. This is a standard conditioning process used in Colloca and Benedetti’s placebo experiments.

The final condition was one in which the participants were told that the green light would be paired with an analgesic just before the shock was delivered – the subjects were told ‘that a green light would anticipate a stimulus that was made analgesic by delivering a sub-threshold electrical shock on their middle finger. Conversely, a red light would anticipate the deactivation of this electrode and thus a painful stimulation on the dorsum of the hand. Actually, all the stimuli were set to go off at the same time as the light.’

What were the results? Quite startling, actually! The subjects who had observed the analgesic effect in the demonstrator rated the green-stimuli consistently less painful than the red-stimuli. And every single green-stimulus was rated lower than the red. This effect simply from watching someone else apparently receiving an analgesia – when actually nothing was being delivered.

The experiential group, those that went through the conditioning procedure themselves, also reported reduced pain when the electric shock was paired with a green light. And finally those who were given a verbal instruction that they would experience analgesia paired with the green light also reported lower pain, but this dropped off fairly quickly after the initial instruction.

So there you have it – somehow by watching someone else obtain an effect, these participants developed a strong and sustained analgesic effect. What is it they were seeing? We’re not sure yet – but Colloca and Benedetti suggest that empathy has something to do with it, because there was a relationship between empathy and the response as measured on the Empathic Concern subscale of the Interpersonal Reactivity Index, a measure often used to investigate trait empathy. This wasn’t demonstrated for other subscales of the IRI.

What can we learn from this? Well, firstly it’s important to recognise that this is an experimental situation in a lab with volunteers – all female – who may not be like you or me! But findings like this can suggest that when we observe someone else reporting and behaving as if a treatment provides good results, we are likely to have a similar effect, provided of course we’re high in empathy. Similarly, but not quite as strongly, we respond to being conditioned ourselves to experience analgesia through a placebo.

Maybe an experiment like this will see the end of celebrity endorsement of magnetic underlays for the bed?!

Colloca, L., & Benedetti, F. (2009). Placebo analgesia induced by social observational learning Pain DOI: 10.1016/j.pain.2009.01.033
Colloca L, Benedetti FPlacebo analgesia induced by social observational learning, PAIN (2009),


  1. Hello Bronnie,

    Could you describe the relationship of IRI scores and observed clinical response? Was the relationship specific to the Empathic Concern subscale and was there a specific cut-off score?


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