The left-digit effect describes a cognitive distortion in which the left pair of digits of a number receives greater attention than the right pair of digits. This means, for example, that the number 99 is categorised as higher than the number 89, even though the difference is only one digit.

The left-digit effect is relevant for neuromarketing as it shows that human perception is not always objective and rational. Consumers can tend to be guided by such cognitive distortions when deciding between products and services. Companies can therefore try to utilise the left-digit effect in a targeted manner, for example by designing their prices so that they start with high figures. Due to the left-digit effect, a price of 99 euros is perceived as lower than a price of 100 euros, even though the difference is only one digit. This can lead to customers being more willing to buy the product as they perceive it to be cheaper.