Infants and children—especially younger children—cannot be studied using the same research methods used in studies with adults. Researchers, therefore, have developed many creative ways to collect information about infant and child development. In this section, we highlight some of the methods that have been used by researchers who study infants and older children, separating them into three distinct categories: involuntary or obligatory responses, voluntary responses, and psychophysiological responses. We will also discuss other methods such as the use of surveys and questionnaires. At the end of this section, we give an example of how interview techniques can be used to study the beliefs and perceptions of older children and adults—a method that cannot be used with infants or very young children.
Figure \(\PageIndex{1}\): Children’s games are more than just fun. They can be opportunities for researchers to learn about how children think, feel, and behave. [“Hide and Seek” by srietzke/Flickr is licensed under CC BY-NC 2.0.]
involuntary or Obligatory Responses
One of the primary challenges in studying very young infants is that they have limited motor control—they cannot hold their heads up for short amounts of time, much less grab an interesting toy, play the piano, or turn a door knob. As a result, infants cannot actively engage with the environment in the same way as older children and adults. For this reason, developmental scientists have designed research methods that assess involuntary or obligatory responses. These are behaviors in which people engage without much conscious thought or effort. For example, think about the last time you heard your name at a party—you likely turned your head to see who was talking without even thinking about it. Infants and young children also demonstrate involuntary responses to stimuli in the environment. When infants hear the voice of their mother, for instance, their heart rate increases—whereas if they hear the voice of a stranger, their heart rate decreases (Kisilevsky et al., 2003). Researchers study involuntary behaviors to better understand what infants know about the world around them.
One research method that capitalizes on involuntary or obligatory responses is a procedure known as habituation. In habituation studies, infants are presented with a stimulus such as a photograph of a face over and over again until they become bored with it. When infants become bored, they look away from the picture. If infants are then shown a new picture—such as a photograph of a different face—their interest returns and they look at the new picture. This is a phenomenon known as dishabituation. Habituation procedures work because infants generally look longer at novel stimuli relative to items that are familiar to them. This research technique takes advantage of involuntary or obligatory responses because infants are constantly looking around and observing their environments; they do not have to be taught to engage with the world in this way.
One classic habituation study was conducted by Baillargeon and colleagues (1985). These researchers were interested in the concept of object permanence, or the understanding that objects exist even when they cannot be seen or heard. For example, you know your toothbrush exists even though you are probably not able to see it right this second. To investigate object permanence in 5-month-old infants, the researchers used a violation of expectation paradigm. The researchers first habituated infants to an opaque screen that moved back and forth like a drawbridge (using the same procedure you just learned about in the previous paragraph). Once the infants were bored with the moving screen, they were shown two different scenarios to test their understanding of physical events. In both of these test scenarios, an opaque box was placed behind the moving screen. What differed between these two scenarios, however, was whether they confirmed or violated the solidity principle—the idea that two solid objects cannot occupy the same space at the same time. In the possible scenario, infants watched as the moving drawbridge stopped when it hit the opaque box (as would be expected based on the solidity principle). In the impossible scenario, the drawbridge appeared to move right through the space that was occupied by the opaque box! This impossible scenario violates the solidity principle in the same way as if you got out of your chair and walked through a wall, reappearing on the other side.
Figure \(\PageIndex{2}\): Even when infants are so young that they have very little motor control, researchers can observe and record involuntary responses, such as an infant’s eye movements, to get valuable insight into what they understand about the world around them. [“The look” by Jerald Guillermo/Flickr is licensed under CC BY-NC-ND 2.0.]
The results of this study revealed that infants looked longer at the impossible test event than at the possible test event. The authors suggested that the infants reacted in this way because they were surprised—the demonstration went against their expectation that two solids cannot move through one another. The findings indicated that 5-month-old infants understood that the box continued to exist even when they could not see it. Subsequent studies indicated that 31⁄2- and 41⁄2-month-old infants also demonstrate object permanence under similar test conditions (Baillargeon, 1987). These findings are notable because they suggest that infants understand object permanence much earlier than had been reported pre- viously in research examining voluntary responses (although see more recent research by Cashon & Cohen, 2000).
Voluntary Responses
As infants and children age, researchers are increasingly able to study their understanding of the world through their voluntary responses. Voluntary responses are behaviors that a person completes by choice. For example, think about how you act when you go to the grocery store: you select whether to use a shopping cart or a basket, you decide which sections of the store to walk through, and you choose whether to stick to your grocery list or splurge on a treat. Importantly, these behaviors are completely up to you (and are under your control). Although they do not do a lot of grocery shopping, infants and children also have voluntary control over their actions. Children, for instance, choose which toys to play with.
Researchers study the voluntary responses of infants and young children in many ways. For example, developmental scientists study recall memory in infants and young children by looking at voluntary responses. Recall memory is memory of past events or episodes, such as what you did yesterday afternoon or on your last birthday. Whereas older children and adults are simply asked to talk about their past experiences, recall memory has to be studied in a different way in infants and very young children who cannot discuss the past using language. To study memory in these subjects, researchers use a behavioral method known as elicited imitation (Lukowski & Milojevich, 2016).
Figure \(\PageIndex{3}\): Filling your basket at the supermarket is an example of a voluntary response behavior. The choices you make are under your control. [“tomatoes” by ikayama/Flickr is licensed under CC BY-NC-SA 2.0.]
In the elicited imitation procedure, infants play with toys that are designed in the lab to be unlike the kinds of things infants usually have at home. These toys (or event sequences, as researchers call them) can be put together in a certain way to produce an outcome that infants commonly enjoy. One of these events is called Find the Surprise. As shown in Figure \(\PageIndex{4}\), this toy has a door on the front that is held in place by a latch—and a small plastic figure is hidden on the inside. During the first part of the study, infants play with the toy in whichever way they want for a few minutes. The researcher then shows the infant how make the toy work by (1) flipping the latch out of the way and (2) opening the door, revealing the plastic toy inside. The infant is allowed to play with the toy again either immediately after the demonstration or after a longer delay. As the infant plays, the researcher records whether the infant finds the surprise using the same procedure that was demonstrated.
Figure \(\PageIndex{4}\): The two-step event sequence Find the Surprise. Left, All of the toys needed to complete the event. Center, A hand flips the latch out of the way so the door can be opened (step 1). Right, A hand opens the door, ultimately revealing a plastic figurine hidden inside (step 2). [These images are from a larger figure initially published in Lukowski and Milojevich (2013). Reprinted with permission.]
Use of the elicited imitation procedure has taught developmental scientists a lot about how recall memory develops. For example, we now know that 6-month-old infants remember one step of a 3-step sequence for 24 hours (Barr et al., 1996; Collie & Hayne, 1999). Nine-month-olds remember the individual steps that make up a 2-step event sequence for 1 month, but only 50% of infants remember to do the first step of the sequence before the second (Bauer et al., 2001, 2003; Carver & Bauer, 1999). When children are 20 months old, they remember the individual steps and temporal order of 4-step events for at least 12 months—the longest delay that has been tested to date (Bauer et al., 2000).
Psychophysiology
Behavioral studies have taught us important information about what infants and children know about the world. Research on behavior alone, however, cannot tell scientists how brain development or biological changes impact (or are impacted by) behavior. For this reason, researchers may also record data about psychophysiological responses, such as heart rate, hormone levels, or brain activity. These measures may be recorded by themselves or in combination with behavioral data to better understand the bidirectional relations between biology and behavior.
One manner of understanding associations between brain development and behavioral advances is through the recording of event-related potentials, or ERPs. ERPs are recorded by fitting a research participant with a stretchy cap that contains many small sensors or electrodes. These electrodes record tiny electrical currents on the scalp of the participant in response to the presentation of particular stimuli, such as a picture or a sound (for additional information on recording ERPs from infants and children, see DeBoer et al., 2005). The recorded responses are then amplified thousands of times using specialized equipment so that they look like squiggly lines with peaks and valleys. Some of these brain responses have been linked to psychological phenomena. For example, researchers have identified a negative peak in the recorded waveform that they have called the N170 (Bentin et al., 2010). The peak is named in this way because it is negative (hence the N) and because it occurs about 140 ms to 170 ms after a stimulus is presented (hence the 170). This peak is particularly sensitive to the presentation of faces, as it is commonly more negative when participants are presented with photo- graphs of faces rather than with photographs of objects. In this way, researchers are able to identify brain activity associated with real-world thinking and behavior.
The use of ERPs has provided important insight as to how infants and children understand the world around them. In one study (Webb et al., 2006), researchers examined face and object processing in children with autism spectrum disorders, those with developmental delays, and those who were typically developing. The children wore electrode caps and had their brain activity recorded as they watched still photographs of faces (of their mother or of a stranger) and objects (including those that were familiar or unfamiliar to them). The researchers examined differences in face and object processing by group by observing a component of the brainwave they called the prN170 (because it was believed to be a precursor to the adult N170). Their results showed that the height of the prN170 peak (commonly called the amplitude) did not differ when faces or objects were presented to typically developing children. When considering children with autism, however, the peaks were higher when objects were presented relative to when faces were shown. Differences were also found in how long it took the brain to reach the negative peak (commonly called the latency of the response). Whereas the peak was reached more quickly when typically developing children were presented with faces relative to objects, the opposite was true for children with autism. These findings suggest that children with autism are in some way processing faces differently than typically developing children (and, as reported in the manuscript, children with more general developmental delays).
Figure \(\PageIndex{5}\): Special equipment has been developed to allow researchers to record the brain activity of very young and very small research subjects. [“Infant EEG Cap Mannequin” by Matt Cain/Flickr is licensed under CC BY-NC-SA 2.0.]
Parent-Report Questionnaires
Developmental science has come a long way in assessing various aspects of infant and child development through behavior and psychophysiology—and new advances are happening every day. In many ways, however, the very youngest of research participants are still quite limited in the information they can provide about their own development. As such, researchers often ask the people who know infants and children best—commonly, their parents or guardians—to complete surveys or questionnaires about various aspects of their lives. These parent-report data can be analyzed by themselves or in combination with any collected behavioral or psycho- physiological data.
One commonly used parent-report questionnaire is the Child Behavior Checklist (CBCL) (Achenbach & Rescorla, 2000). Parents complete the preschooler version of this questionnaire by answering questions about child strengths, behavior problems, and disabilities, among other things. The responses provided by parents are used to identify whether the child has any behavioral issues, such as sleep difficulties, aggressive behaviors, depression, or attention deficit/hyper- activity problems.
A recent study used the CBCL-Preschool questionnaire (Achenbach & Rescorla, 2000) to examine preschooler functioning in relation to levels of stress experienced by their mothers while they were pregnant (Ronald et al., 2011). Almost 3,000 pregnant women were recruited into the study during their pregnancy and were interviewed about their stressful life experiences. Later, when their children were 2 years old, mothers completed the CBCL-Preschool questionnaire. The results of the study showed that higher levels of maternal stress during pregnancy (such as a divorce or moving to a new house) were associated with increased attention deficit/ hyperactivity problems in children over 2 years later. These findings suggest that stressful events experienced during prenatal development may be associated with problematic child behavioral functioning years later—although additional research is needed.
Figure \(\PageIndex{6}\): Parents spend countless hours together with children observing their behavior. Developmental psychologists sometimes use surveys to collect information from parents that can be used to answer important research questions. [“DSC01065_1” by Jerry Lai/ Flickr is licensed under CC BY-NC-SA 2.0.]
Interview Techniques
Whereas infants and very young children are unable to talk about their own thoughts and behaviors, older children and adults are commonly asked to use language to discuss their thoughts and knowledge about the world. In fact, these verbal report paradigms are among the most widely used in psychological research. For instance, a researcher might present a child with a vignette or short story describing a moral dilemma, and the child would be asked to give their own thoughts and beliefs (Walrath, 2011). For example, children might react to the following:
Mr. Kohut’s wife is sick and only one medication can save her life. The medicine is extremely expensive and Mr. Kohut cannot afford it. The druggist will not lower the price. What should Mr. Kohut do, and why?
Children can provide written or verbal answers to these types of scenarios. They can also offer their perspectives on issues ranging from attitudes toward drug use to the experience of fear while falling asleep to their memories of get- ting lost in public places—the possibilities are endless. Verbal reports such as interviews and surveys allow children to describe their own experience of the world.
