In our everyday lives we constantly compare things. We care about whether we are better or worse off than others around us, or than we were in the past.
Why we do this has long puzzled scientists, because a rational being who sees the world in an objective and absolute way ought to be more successful. But in a study published in the journal Science, we recently showed that such comparative judgements can actually give individuals an evolutionary advantage.
The common tendency to compare is strikingly illustrated by “contrast effects”, where the response to a given stimulus is altered by other available stimuli, or by experiences in the recent past. Contrast effects change our perception of the world.
The famous checker shadow illusion, in the image below, demonstrates this effect.
Researchers on consumer behaviour have long known about contrast effects and regularly exploit them to influence shopping habits. Place a slightly cheaper item next to a more expensive one and, experiments show, the cheaper item will sell much more.
And it is not just humans that are susceptible to contrast effects. In the 1940s, psychologist Leo Crespi trained laboratory rats to run along a passageway to reach a food reward at the end. The rats ran more slowly for a small amount of food if they had previously been used to getting larger rewards, compared to control rats always rewarded with the smaller amount.
Coping with change
Insects, birds and mammals all show contrast effects, but the reason for this bias has remained a mystery. Our new study provides a possible explanation, by showing that sensitivity to past experiences may be an evolutionary adaptation to a complex, changing world.
John McNamara, Alasdair Houston and I used mathematical tools to understand how an animal should respond when environmental conditions change over time. We were specifically interested in environments that are “autocorrelated”, which is where the conditions experienced today tend, on average, to be similar to the conditions experienced tomorrow. Most natural environments are autocorrelated in this way. For instance, it is statistically more likely to rain tomorrow if it was raining today.
In such circumstances, past experiences may give valuable information about how the world is changing, and hence how things are likely to be in the future. An animal that has previously experienced poor conditions learns that the world is generally an unfavourable place, so has low expectations about the future. This means that if a good opportunity comes along, it should respond much more strongly.
Our mathematical model formalised the logic of this idea. We calculated the optimal behaviour for an animal that has to work for food under changing conditions, just like the rats in the classic experiments. We assumed that the animal is unsure whether the world is harsh or benign, but updates its belief by learning from conditions experienced in the past. Our model confirms that optimal behaviour will lead to contrast effects, because animals used to poor conditions have lower expectations of the future than those used to rich conditions.
Influenced by the Angels
Contrast effects have been known for some time in studies of mate choice and attractiveness. For example, how keen you are to date someone is influenced by the attractiveness of other people you have seen in the recent past. This makes good evolutionary sense, because you should be less willing to settle down with someone if your past experience tells you that there are lots of better options around.
The effect was illustrated by a famous experiment in the 1980s, run by psychologists Doug Kenrick and Sara Gutierres. They asked male university students to rate the attractiveness of a woman in a photo, which was shown to them either before or during an episode of the original Charlie’s Angels TV series.
Kenrick and Gutierres found that the students gave lower ratings to the woman in the photo while they were watching Charlie’s Angels, whereas a control group who watched a different TV show gave similar ratings before and during that show. This fits with a negative contrast effect: people respond less to a test stimulus (the woman in the photo) if they have recently been exposed to stimuli of higher value (the Charlie’s Angels actresses).
Our mathematical model extends this well-known result beyond mate choice to show that contrast effects are a logical outcome of the way we respond to the world around us.
Related research in our group in Bristol has shown that several other psychological biases, including optimism, pessimism and the placebo effect, are also adaptive when conditions change over time. This work is teaching us that many curious features of the human mind may be deeply rooted in our evolutionary history, helping us to cope with a complex, unpredictable world.