Why do babies put everything in their mouths?

Spend enough time around babies and you’ll notice they’ll put just about anything into their mouth. Many people pass this off as an infant’s way of feeling their surroundings. Yet, this argument does not capture the science behind the world’s cutest phenomenon.

It’s a common scene for any new parent –

Their baby, happily entertained on the floor, grabs a close-by object. This can be anything, from the Sunday newspaper to a chew toy  the dog just dropped. After waving it about to test for structural integrity, said object goes straight into their mouth.

There is often no selectivity about cleanliness, texture, scent, or even taste, much to the dismay or embarrassment of the parent.

Some careful observers may come to the correct conclusion that this type of behavior is instinctual. But, why would a baby be biologically programmed to mouth anything within reach?

The simplest and most common explanation is that a baby lacks the fine motor control needed to probe their surroundings with fingers. Without hands, they resort to their next most dexterous sensor- tongues.

However, this explanation doesn’t stand up to scientific scrutiny.

Many of the things a baby mouths are potentially harmful. From various pet residues to common household plants, these substances carry harmful bacteria or viruses that could infect an infant, or toxins that are poisonous.

Obviously, babies do not consciously know what is good or bad for them, but, evolution over millions of generations does. Humans are incredibly adept at sensing substances that may harm us. The smell of bodily excretions or the bitter taste of many poisons tends to repel us.

The next obvious hypothesis is that babies simply do not develop keen taste buds until later. Wrong again – the sense of taste is fully developed within a few weeks of birth. In fact, some research suggests that babies have an even greater ability to perceive tastes than adults. Taste just doesn’t seem to be a factor in normal mouthing.

Instead, instinctive mouthing is better explained by the “Hygiene Hypothesis.” First proposed in 1989, the hygiene hypothesis theorizes that repeated exposures to infectious agents during infancy allows the immune system to develop and later guards children against various autoimmune disorders, such as asthma and Crohn’s Disease.

So why does the hygiene hypothesis have more credibility than the idea that the mouth is just used as an exploratory tool?

Firstly, infants have a turbo-charged immune system. They have a higher number of white blood cells than adults – specifically the number of leukocytes and lymphocytes (B cells and Natural Killer cells) are all increased in an infant’s body. So while babies don’t have any immune memories, they are better able to respond to new infections.

For babies, coming into contact with as many news things as possible is like cramming for a test. By giving their elevated immune systems a chance to read all these new substances, they are preventing more diseases in the future.

But why the mouth?

While it may come as a surprise, our immune system develops mostly in our gut. Most bad things we are exposed to actually come through our stomach, where acid renders them harmless. Following partial digestion by stomach acid, our immune cells grab onto those biological pieces and remember them in case something similar tries to attack in the future.

Essentially, it comes down to this: babies try to put anything and everything in their mouths to maximize immune system development. This gives them a better chance at being healthy later on.

Of course, there are things around like pesticides or medicine that babies were never meant to chew – but these things were not around millions of years ago when infants were evolving the instinct to mouth everything. Instead, stone age babies were more likely to mouth something their siblings had just used, thereby giving it protection to the most common pathogens in the area. In fact, a recent study showed that parents who cleaned their infants pacifier by sucking on it instead of boiling it had children with much lower rates of asthma or eczema.

This evidence corroborates the accepted theory that cleaner babies are actually sicker later in life. The best known example is that clean babies are much more likely to develop asthma.

So, the next time you go to take something out of a babies’ mouth, let them chew it if it isn’t dangerous. It may just help out later.

Image: The cutest baby picture available online.

Edit: Wording change for clarity

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