Just Noticeable Difference (JND) in Psychology

The just noticeable difference (JND), also known as the difference threshold, is the minimum level of stimulation that a person can detect 50% of the time. For example, if you were asked to hold two objects of different weights, the just noticeable difference would be the minimum weight difference between the two that you could sense half of the time.

The just noticeable difference applies to all senses including touch, taste, smell, hearing, and sight. It can apply to things such as brightness, sweetness, weight, pressure, and noisiness, among other things.

It is important to understand the just noticeable difference vs. the absolute threshold. While the difference threshold or the just noticeable difference between two stimuli means detecting differences in stimulation levels, the absolute threshold refers to the smallest detectable level of stimulation.

The absolute threshold for sound, for example, would be the lowest volume level that a person could detect. The just noticeable difference would be the smallest change in volume that a person could sense.

History of the JND Concept

The difference threshold was first described by a physiologist and experimental psychologist named Ernst Weber and later expanded upon by psychologist Gustav Fechner. Weber's Law, also sometimes known as the Weber-Fechner Law, suggests that the just noticeable difference is a constant proportion of the original stimulus. However, we now know the JND is not a constant, but rather, a variable.

How Just Noticeable Difference Is Measured

In order to measure sensory information and difference thresholds, scientists rely on responses from study participants. The measurements scientists take depend on the sensory information being received. For instance, when measuring sound, they may use decibels. When measuring heat, they may use joules.

The JND is usually determined by conducting multiple trials and then using the smallest levels that participants could detect at least 50% of the time.

Neural Activity

Our brains receive sensory input (such as light, sound, or taste), and our sensory organs convert this input into electrical signals received by the brain. Therefore, measuring neural activity is another way in which researchers may determine how much (and what type of) sensory input a person is receiving.

In one study, researchers found they could distinguish whether or not someone was touching something sticky based on their neural activity.

Stimulus Intensity

The intensity level of the stimulus can also play a role in how much people notice changes. If a light is very, very dim, people might be more likely to notice smaller changes in intensity than they would if those same changes were made to brighter light.

For example, imagine that you are in a dark movie theater. The house lights slowly start to turn on and you immediately notice even a very small change in the light intensity. Afterward, you leave the theater and head outside where the sun is shining brightly. If the same changes in light intensity were made outside, you might be less likely to notice them since the stimulus level is much higher.

Examples of Just Noticeable Difference 

The following are everyday examples in which the just noticeable difference can be observed.

Weight

Imagine that you volunteer for a psychology experiment at your school. The researchers ask you to hold two small amounts of sand in each hand. An experimenter slowly adds tiny amounts of sand to one hand and asks you to say when you notice that one hand feels heavier than the other. The smallest weight difference that you can detect at least half the time is the just noticeable difference.

Sound

You are watching television with your partner, but the volume is too low to hear. You ask your partner to turn it up. They press the volume button twice, but you still cannot tell a difference in the volume. Your partner presses the button two more times before you are able to notice the increase in volume.

Or: You are having a party at your apartment and the neighbor comes over and asks you to turn the music down. When you do, you and your guests immediately notice that the music is much quieter, but your neighbor does not notice a difference in the volume because the change is below their difference threshold.

Taste

You volunteer for another psychology experiment at your school. This time, the experimenters place small amounts of sugar in a container of water and ask you to drink it. You are asked to say when you notice the sweetness of the water versus the plain water. The smallest level of sweetness you can taste half the time is the difference threshold.

Color

You dye your hair, but afterward, the color still looks the same as it was. This is because you dyed your hair a similar shade to what you already had, and the color isn't above the difference threshold.

Just Noticeable Difference in Marketing

Marketers may take advantage of the JND in several ways. For instance, a company may determine the JND of a price point so they can increase the price of a product ever so slightly so that people won't notice it.

Companies often reduce the size of packages, such as boxes of pasta or cans of corn. They'll knowingly decrease the size by an amount that is below the difference threshold, so people don't realize that their favorite products have actually gotten smaller.

Downsizing packaging saves companies money, but it's often seen as an unethical practice—"tricking" the consumer into buying their favorite products at the same (or higher) prices, but they're getting less without knowing.

Why Just Noticeable Difference Matters

Just noticeable difference is part of a field of study known as psychophysics. Psychophysics examines how physical stimuli in the environment affects and interacts with mental processes. In other words, while scientists used to focus on measuring objective data, psychophysics allows them to measure subjective experiences as data.

The JND is often studied in product development. For instance, when manufacturing cell phones, companies measure the JND of volume so that there are noticeable increases when you raise the volume on your device.

Companies that create food products also study the JND. For instance, if a product is supposed to taste like green apple, the manufacturer needs to know the minimum amount of green apple flavor required so that most people will be able to taste it.