Let your baby choose: understanding your infant's preferences

Age range: 0 to 6 months

What you'll need A pacifier that your infant will suck on for about 15 minutes at a time; the operant conditioning web tool available here

In this lab, you’ll let your baby control what sound is played by sucking faster or slower on a pacifier. We recommend starting by trying to observe his or her preference for hearing music or a heartbeat—but you can also try uploading your own sound files and let your baby choose between them!


It can be hard to tell what a newborn likes and doesn’t like: this is one of the main challenges parents face in those sleepless first months. Some preferences are pretty clear: babies generally like milk,warmth, and being held. But did you know that even newborns can demonstrate complex preferences for…

These preferences were all demonstrated using a low-tech technique from developmental psychology called “operant sucking.” In this form of conditioning, infants are rewarded with a particular type of image or sound when they suck on a pacifier more often than they do at baseline. If infants learn to suck faster under these conditions and if they can also learn to suck slower when the reward is switched to being provided then—they are showing a preference for the “reward.” In one study, infants actually showed a preference for the specific story their mothers had read to them in the womb over unfamiliar stories (DeCasper & Spence, 1986)!

Many other techniques are used to assess infants’ preferences and to find out what differences they can detect, well before they’re able to talk. These include other operant techniques (where something depends on the infant’s behavior), like conditioned head-turn or conditioned looking protocols, as well as measures based just on the amount of time an infant looks at a particular image. Operant sucking, however, is particularly well-suited to try at home—you don’t need any special equipment or a careful estimate of where your child is looking.


Try this lab when your infant is alert but calm and not hungry.
  1. Go to www.mit.edu/~kimscott/pacifier.html Give your infant a pacifier and measure his or her baseline sucking rate for at least 2 minutes. Click “Begin baseline measurement” to begin, then click the pacifier button each time your child sucks the pacifier. Infants tend to suck in short “bursts”; the web tool will measure the times in between these bursts of sucking (defined as the gaps greater than two seconds) and plot them at the right. Once the status indicator turns blue you can click “End baseline measurement.”
  2. Now you can start "conditioning" him or her to suck faster or slower. Choose which sound will be played when your baby sucks faster (SOUND 1) and which sound will be played when your baby sucks slower (SOUND 2). You can choose from a heartbeat, some music, or silence for each sound, or (coming soon!) upload your own files. We recommend starting by setting SOUND 1 to either music or a heartbeat and SOUND 2 to silence. This means that when your baby sucks more quickly than usual, he or she will hear the music or heartbeat, but when she sucks more slowly, there will be no sound.
  3. Now click “Start conditioning” and continue pressing the green button each time your child sucks on the pacifier. When your child is sucking faster than usual, SOUND 1 will automatically be played. When your child is sucking slower than usual, SOUND 2 will automatically be played. After 5 to 10 minutes, you may notice that his or her inter-burst intervals have either increased--sucking faster to hear SOUND 1--or decreased--sucking slower to hear SOUND 2.
  4. There are many reasons a child's sucking rate might change over the course of this experiment besides a preference for one sound or another. To see if your child is really showing a preference, switch SOUND 1 and SOUND 2. Does your child’s sucking rate now decrease if it increased before?

To read more:

About infants’ preference for hearing a heartbeat sound, assessed using operant sucking:

About infants’ preference for the stories their mothers read during pregnancy:

Other examples of findings using operant sucking procedures: