We make quick guesses about quantity all the time, from judging the length of a queue to glancing at a pile of coins. These snap estimates feel automatic and objective. A new study from China suggests they are not, and that we tend to underestimate how many things there are when those things make us feel good.
Researchers at Suzhou University of Science and Technology ran three behavioural experiments. The findings appear in BMC Psychology. In each experiment, young adults aged 18 to 25 compared two groups of items flashed briefly on a screen and judged which group contained more.
Number sense is the ability to recognise roughly how many items are present without counting. It is thought to be a basic feature of vision, much like seeing colour or contrast, and largely independent of feelings or beliefs. The study tested whether that independence holds when the items carry positive meaning.
In the first experiment, 30 people compared arrays made up of happy, neutral or sad faces. Arrays of happy faces had to contain more faces than the others before people judged them to be equal, which shows they were consistently underestimated. Neutral faces were also judged to be fewer than sad faces.
The second experiment, with 32 people, repeated the test without a fixed comparison group, and happy faces were still underestimated compared with sad ones. In the third, 34 people compared arrays made of images of Chinese banknotes with arrays of plain rectangles. The money arrays were underestimated too, which suggests the effect extends to reward as well as emotion.
The items appeared for only a fraction of a second, and participants said afterwards that they could hardly tell what type of image they had seen. This suggests the bias was not the result of deliberate guessing. The researchers explain the effect using predictive processing theory, which proposes that the brain constantly forms expectations about the world and adjusts what we perceive to match.
The findings matter because errors in judging quantity can feed into everyday decisions, from spending and saving to how much food we serve ourselves. If rewarding things seem fewer than they are, people may be tempted to seek more of them. Marketers and designers may also be able to shape such judgements without people noticing.
The study has limits. The experiments used simple images in a laboratory and recruited only young adults, so the results may not apply to children, older people or real world settings. Colour, contrast and density were randomised rather than fully controlled, which may have added noise.
Number sense has long been seen as a pure, low level perceptual skill. This research adds to evidence that emotion and reward can quietly colour even our most basic judgements.
