Baby Cry: Annoying Sound or Important Communication?

Have you ever been stuck in the same room with a crying baby? If you have, chances are you know how difficult it can be to make the baby stop crying. Or at least you know how difficult it can be for you to make him stop. Babies are soothed by their mothers more easily than anyone else. The baby might keep crying even when you give him his favorite toy, but he will calm down when his mother does the same. Do not worry, it is nothing personal—it is just the way babies’ brains are wired! Because babies are so dependent on their parents, human beings grow close to their families very early in life. For example, imagine a puppy playing with her mom. Because the mom protects the puppy from potential danger, the puppy can grow up to start her own family. Once the puppy grows up and becomes a mother, this dog then protects the new babies until they grow up to make even more puppies. By protecting the youngest members of the family, animals guarantee the continued existence of their species. For this reason, mother–child relationships in many species are crucial to a baby’s healthy development: their bond keeps the species alive. After all, who wants to live in a world without puppies?

One of the ways parents protect their children is by carrying them. Think back to that crying baby. How does his mom make him stop? She knows that babies only cry when they need something, so she tries to figure out what the baby wants. As mom scrambles to find the problem, she picks the baby up. The crying stops almost immediately. As parents have known for years, babies calm down when they are carried by their mothers [1]. However, it was not until recently that we learned why this happens.

Why Does Carrying a Crying Baby Calm Them Down?

When babies are picked up, there are natural reactions in their bodies that help them calm down. Generally, babies will relax into their mothers’ arms, making them easier to hold. When babies are picked up by their moms, their heart rates drop—a sign of relaxation. Any time a person becomes panicked, the heart rate speeds up. A crying baby has a high heart rate because he wants his mom. But when mom picks up the baby, these signs of panic decrease. Researchers noticed that babies being carried have lower heart rates than before they were picked up, which means that they are calmer than they were before. In other words, when a mom picks up her baby, the child cries less, moves less, and relaxes more. Of course, this will work for your little sister or brother, but it does not just happen in humans. This pattern has been studied in other animals too, and it is called the transport response (TR). Healthy young animals relax when they are being carried. This is a normal TR.

Experiment with Human Babies

To find the mechanisms in the brain that make babies stop crying, experimenters tested a group of healthy babies and their mothers. Children were monitored in three different stages: in the crib, mom holding the baby, and mom carrying the baby. Babies left in a crib will usually start crying to protest being separated from their mothers. When the babies started to cry, researchers asked the mothers to pick up their babies. These mothers sat in a chair and held their babies for 30 s (holding condition, Figure 1A). Then they were asked to walk around, still holding the babies (carrying condition, Figure 1A). This lasted another 30 s. In this final stage, the babies relaxed into the embrace and stopped crying. These children reacted to their moms as soon as their mothers picked them up and started to walk around. All of them relaxed into their mothers’ arms, slowed their heart rates, and stopped crying (Figure 1B). A representation of these changes is shown in Figure 1B. In the upper part of the graph, the vertical lines represent when the baby moves; in the middle of the graph, the horizontal bars represent when the baby cry; and in the lower part, the up-and-down continuous line represents the distance between two heart beats [Inter-Beat-Interval (IBI)]. The slower the heart, the higher the interval between two heart beats. Furthermore, the blue lines in the plot represent babies’ reactions to holding, while the red parts represent babies’ reaction to carrying. During holding, (blue) babies move and cry a lot and the interval between two heart beats is low; therefore, the heart rate goes fast. On the contrary, during carrying, (red) babies move less and do not cry and the interval between two heart beats is longer, meaning the heart rate goes slow. Carrying was the most effective soothing method. Leaving the babies in the crib (crib condition) had no results, and holding was less effective than carrying. Indeed, during holding, it took much longer for the babies to relax and stop crying compared to carrying.

Does Carrying Work Only for Humans?

In another test, researchers did similar experiments with mice instead of people [2]. These scientists placed baby mice in a cup, a few inches away from their mother. The babies began to cry about being separated from their moms, who came to rescue them immediately. When the moms picked up their babies to take them out of the cup, researchers watched the pups’ physical reaction. They found that mice react similar to humans, since the baby mice usually stopped moving when they were being carried. When babies stop moving, this makes them easier to transport, which helps the mother.

How Does the TR Work?

Because mice walk on all fours, they cannot hold babies in their arms the way humans do. Instead, the mother mouse uses her mouth to grasp a bit of skin at the back of the mouse pup’s neck (Figure 2). Researchers were able to recreate this grip by pinching this same spot [3–6]. This way, the researchers had more control over the experiment. In the first mouse experiment described earlier, the researchers could not record all aspects of the pup’s reaction, like heart rate. They also had to wait for the mother to come rescue her pups. In this new, controlled experiment, they were able to monitor the animals more closely. The researchers carried the pups by the skin on the back of the neck and watched their reactions. The pups acted a lot like they did when they were carried by their mother. Whether carried by an experimenter or their mother, the mouse pups dramatically relaxed during transport. As we also saw in humans, their heart rates fell quickly and they stopped crying. Because the mouse pups and the human babies reacted similarly, researchers began to use mice to better understand people.

Surprised that a researcher and a mouse mother had similar effects on the mouse pups, scientists began to wonder how the brain controls the TR. They knew for certain that the mouse pups relaxed when carried by a mother or a researcher. When either of these things happened, the pups felt the grip of whoever was carrying them and could sense their distance from the ground. This awareness controlled their TR. To understand how the TR is triggered, researchers gathered a new group of mice. These animals had a mutation, which means that they had a slight change in their DNA to make them different than normal mice. The DNA that was different in these mice was responsible for the development of the cerebellum. The cerebellum is the part of the brain that controls movement, in both mice and humans. When this group of mutant mice was tested, they did not react to being carried. Instead, these pups continued to squirm. All of the normal mice had frozen in place when lifted off the ground. But the mutant mice did not react like the others (Figure 3). Those who were squirming clearly showed the effects of their mutation.

This was another clue that the TR depends largely on the health of an animal or person, and this clue can help scientists to understand some disorders that affect the way an animal interacts with others. Because the mutant mice kept moving under the same conditions that caused the normal mice to become still, researchers believed that the lack of reaction to being carried was the result of cerebellum dysfunction. If a mouse moves when it is not supposed to, this movement comes from the cerebellum. Scientists think that similar signals from the cerebellum might control whether human babies stop crying when carried, too.

Finally, to study the change in heart rate that occurred during the experiment, researchers watched the part of the brain that controls the heart. This is called the autonomic nervous system (ANS). If you want an easy way to remember the ANS, think of the word automatic. The two words, autonomic and automatic, sound very similar, because they mean the same thing! The ANS is responsible for all the things that happen in your body that you do not have to think about. Your body reacts to some things without you having to control it. Even though you do not think about your heart rate, or remind yourself to breathe, these things happen automatically. These functions are all controlled by the ANS. The ANS has two parts. They are called the sympathetic and parasympathetic branches. The sympathetic nervous system (SNS) controls “fight or flight” or panic reactions. When we are very scared, our bodies get ready to run or to defend themselves, which all happens because of the SNS. The parasympathetic nervous system (PNS) calms people down when they are no longer in danger. When a baby cries about being separated from his mother, the panic system is at work, meaning the SNS is active. However, when the mother picks the baby up, the baby’s PNS calms him down by decreasing his heart rate. A lower heart rate means that the baby is calmer, so as his body returns to normal, and the baby feels secure and safe.

In the experiment with mice, researchers injected two different kinds of drugs that blocked one of the two branches of the ANS. The first drug blocked the SNS. After the injection, there was no change in the heart rate response to being carried, carrying still slowed down the heart. However, when the same test was done after blocking the PNS, the heart rate no longer slowed down in response to being carried. This proved that the PNS is responsible for the baby’s TR.

The science behind this discovery can improve people’s lives. One way that this research will make a difference in the world is by teaching parents how to soothe their babies. When a baby did not stop crying, parents sometimes get very frustrated. For this reason, unstoppable crying is a leading cause of child abuse. The safer and easier it is to relax a baby, the less likely it is that innocent babies will be hurt by frustrated parents.

Why Understanding Younger Siblings’ Cry is Important?

In this study, we learned babies stop crying when they are carried by their mothers. Researchers found that carrying calms babies, no matter what kind of babies they studied. From this research, we now know that animals physically relax when they are held by a walking mother. What is more, researchers discovered the mechanisms behind this calming response. With the knowledge that the parasympathetic branch of the ANS and the cerebellum control the TR, we now understand what happens in a baby’s brain when the baby is carried by his caregiver. These behaviors, both maternal behaviors and the following babies’ relaxation, help to keep our species alive. Furthermore, this study may help psychologists who are doctors trying to help people live better if the brain does not function completely well. The better doctors understand the healthy mind, the better they can diagnose brain problems in children who have them. Knowing about the normal response that happens when moms carry their babies, we can use the response to maternal carrying as an early test to show signs of certain disorders, like autism [8]. Since some children do not physically respond to being carried, psychologists have begun to explore these irregular interactions between parents and children to see if they predict future brain disorders. For example, a child who is developing normally relaxes into his mother’s embrace when she carries him. But some parents report feeling “as if (they) were holding a stone or a sack of flour, not a baby,” when their child is crying [5]. These were the parents of children who were not physically adjusted to fit parental carrying. If a baby does not react with the TR when carried, the baby himself/herself seems to be heavier because it is difficult to carry him/her.

Interactions with parents are crucial to every child’s development. By teaching parents and children how to interact better with each other, scientists can improve communication between family members. Also, everyone can help this process by learning and sharing information like this. As authors, we chose to explain this discovery to a young audience because of how important it is to the future generations. The more we know about health and the brain, the better we understand ourselves. When parents know what controls the actions of a baby, they learn how to connect with that baby. At the same time, as scientists learn more about the brain, they can train parents on how to better interact with their children and to aid in healthy development.

Glossary

Transport response (TR): ↑ When a baby is carried, he/she shows a specific set of responses, which calm him/her down (fewer movements and cry, slower heart rate) and which is called Transport Response.

Inter-beat-interval (IBI): ↑ The heart is a muscle that periodically contracts. Each contraction is called a “beat.” The Inter-Beat-Interval is the time, which passes between two beats.

Cerebellum: ↑ The cerebellum is responsible for movement coordination. We can jump or write because the muscles involved are activated in a coordinated way, and the cerebellum is responsible for this coordination.

Autonomic nervous system (ANS): ↑ There are some vital functions, such as breathing, which are continuously done without the need for us to think about them. These functions are regulated by the autonomic nervous system.

Sympathetic nervous system (SNS): ↑ The sympathetic nervous system is the part of the autonomic nervous system, which controls the bodily reactions when we are anxious.

Parasympathetic nervous system (PNS): ↑ The parasympathetic nervous system is the part of the autonomic nervous system that controls the bodily reactions when we are calm and relaxed.

Disclaimer

The authors of this article would like to remind their readers that this discovery does not permit you to carry your siblings without a parents consent. Stay true to your rules at home and tell your parents about what you learned. Above all, be careful with younger siblings. As we just learned, their brains are pretty complicated, so it is important to treat them gently.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.