Why is it that the reaction we get from absurdity is laughter? Everyone does it, even babies. Is there a reason why it is our instinct to laugh when we see or hear something absurd/humorous
Like so much of biology, we just don't know!
I'll preface this answer by disappointing you; this answer doesn't entirely answer your question. That's because this is a pretty big mystery in research and a fascinating topic. I'll start with a couple of quotes that maybe explain why!
"If you search on the Web of Science database for papers on the emotion of fear, you’ll get back 6,477 published papers. Search for papers on laughter and you’ll get a paltry 175."
-Sophie Scott 2015
In other words, the lack of research into laughter possibly is because research usually tries to solve a problem. Generally, laughter isn't perceived as a problem and is relatively seldom investigated. Notably, there is a symptom called the Pseudobulbar affect. It describes someone that laughs (or cries) uncontrollably.
"Analysing humour is like dissecting a frog; few people are interested, and the frog dies of it."
-Elwyn Brooks White 1941
Why do we (as humans) laugh?
The consensus in the community of researchers seems to be that we laugh because it strengthens human social connections.
Counter-intuitively, it's not always because we find things funny. The linked article implies that although it's not exactly clear why we laugh, laughter is a form of seemingly non-deliberate social interaction that doesn't interrupt or replace speech, but adds to it. We laugh in addition to talking.
The evolution of laughter.
As you mention in your question, even babies laugh. Laughter is clearly not unique to humans, let alone human adults. Most social mammals laugh and there are lots of studies on ape laughter and a niche field in "rat tickling" which studies laughter in rats. Jaak Panksepp even found that rats can be bred that love to be tickled! That article also discusses the science surrounding laughter and the links laughter has with increasing neurogenesis. This implies that laughter emerged a long time ago even on an evolutionary timescale and contributes in a very real way to the wiring of our brain. It's not clear why this happens.
There is a popular but unfounded theory put forward by the philosopher John Morreall that laughter originated as some sort of group "the danger is over" signal after the flight or fight response.
Going back a step; what is laughter?
To fully place laughter in the context of our social constructs, we must first understand what laughter is. Unanimously, all types of laughter are non-verbal expressions of emotion (and above there is a reference to how laughter fits in around language, enriching it). These positive non-verbal auditory signals have been shown in primates to be involved with mirroring the emotions of others. At a higher level, there are different types of laughter in our social world ranging from Schadenfreude to laughter based off of specialist interest, all of which seemingly have different triggers, varying numbers of potential participants, and social purposes.
Another question is raised as people can interpret the authenticity of laughter. This study found that people can very easily perceive an inauthentic "polite tittering" from an "explosion of mirth". This highlights that why-ever we laugh, laughter has a complex and under-appreciated social underpinning in humans that we don't yet understand.
To me at least, laughter may be the result of primitive non-verbal emotional communication and lie-detection; part of the intra-species evolutionary arms race of big-brained social mammals in terms of altruism and lying. For example, I would laugh, and I would gauge the state of you by seeing if you laugh too, and then I can act on how I think you feel; be it altruistically or selfishly. You would need to be able to tell if my original laugh was testing you, or genuine.
With that tool as a basic premise, one can imagine how such a wide variety of functions arose from a simple two-party status check.
It is not known for sure. But I find Hurley and Dennett's theory very convincing.
Our brains are engaged full time in real-time (risky) heuristic search, generating presumtpions about what will be experienced next in every domain. This time-pressured, unsupervised generation process has necessarily lenient standards and introduces content - not all of which can be properly checked for truth - into our mental spaces. If left unexamined, the inevitable errors in these vestibules of consciousness would ultimately continue on to contaminate our world knowledge store. So there has to be a policy of double-checking these candidate beliefs and surmisings, and the discovery and resolution of these at breakneck speed is maintained by a powerful reward system - the feeling of humor; mirth - that must support this activity in competition with all the other things you could be thinking about.
I realize that the writing style is dense, and the quote requires some knowledge of their assumptions, so here is an explanation.
Pre-pre-explanation: What is a mental space?
A mental space is one of the modern assumptions of how humans organise the knowledge in their heads. I won't spend time explaining how it differs from other assumptions such as scripts. It is probably best introduced by thinking of fiction. When you watch Star Trek, you can easily think within the "Star Trek universe". You know that within this world, it is possible to beam up people, that money doesn't matter but dilithium crystals do, and so on. Your head holds a description of this world, and you can do logical reasoning to infer things which are true within this world.
A mental space is such a "world" within our head. It is rarely as elaborate as the Star Trek world. It is just a compartmentalization of our knowledge of our surrounding world, such that we can hold contradicting views of the world at the same time. This is useful, for example, when we consider whether we should hurry up for work: we can conjure up a mental space representing our day if we arrive at work on time, then a contradicting one where we arrive late, and compare them.
Pre-explanation: Why we find things funny?
Our heads are constantly carrying around innumerable mental spaces, some we actively think about, others buried in memory. And all the time, we are learning new information, which may or may not be true. It is in our best interest to incorporate true information into appropriate mental models (a mental model is a complex of theories which explain to us how something works), and discard false information. For example, when a Star Trek episode introduces the Tribbles species, we have to incorporate the information that Tribbles exist as true in our "Star Trek canon" mental space and possibly our "What I'd do if I were a Starfleet officer" mental space, and keep it as false in all other mental spaces.
What Hurley and Dennett are proposing is that our brains know how to evaluate information for truth value (or at least consistency with a mental model). This job is, like most jobs in the brain, too computing intensive for the conscious mind. But when it gets done, the conscious mind has to be informed about the computation result. This information arrives, as with all other conclusions of the unconscious computation, in the form of a feeling - in this case, the feeling of mirth. So, if you find something funny, it is a sign that the mental space you just built (which contains this "something") is incongruous and should be abandoned as untrue.
Explanation: Why we laugh?
We are social animals. It will frequently happen that we are all receiving the same new information from outside as our herd comrades. And out in the jungle, we frequently evaluate new information as being a possible danger. Is this rumbling a tiger? When a herd member has computed the signal and his sense of mirth has told him that the "this is dangerous" mental space is internally inconsistent, the automatically triggered laughter will inform other herd members that the situation is safe and they can relax. This is what makes laughter contagious and pleasant (although they have additional arguments for "pleasant" - basically, a reward without which we wouldn't be inclined to go through the resource-hungry heuristic computations).
- We feel something is funny when we consider a proposition which turns out to be untrue,
- especially also when we consider a potential danger and it turns out that we are safe.
- We laugh when something is funny as a signal to others that "things are not what they seem to be",
- especially that a danger we may all be evaluating is not dangerous.
Arguments for the above theory
I only had the space here to present the content of the theory. For the arguments why they believe this theory to hold, and how it compares to multiple competing theories, please see their book. It is a truly fascinating read.
Hurley, M. M., Dennett, D. C., & Adams, R. B. (2011). Inside jokes: Using humor to reverse-engineer the mind. MIT press.
Why do we laugh?
The leading research on why we laugh is done by Robert Provine. He has even written several books on the topic. His theory is that laughter was a primitve form of communication that evolved. For example,he and some graduate students listened in on average conversations in public places and made notes. And in a survey of 1,200 "laugh episodes," he found that only 10%-20% of laughs were generated by anything resembling a joke. The other 80%-90% of comments that received a laugh were dull non-witticisms like, "I'll see you guys later" and "It was nice meeting you, too.".
One reason could be is that it is to make other people nearby feel good and charismatic. It is used to show you like them. It is a social signal[1,2 3]. What would support this is the fact that laughter occurs more in social situations in front of a person.
Another, more older hypothesis is thaf the first human laughter may have begun as a gesture of shared relief at the passing of danger. And since the relaxation that results from a bout of laughter inhibits the biological fight-or-flight response, laughter may indicate trust in one's companions. Again, this is a type of social signal. Plus, this may be the reason why laughter has evolved to be contagious. Again, the fact that laughter occurs in a social situation supports this hypothesis as well.
Note that laughter isn't just a human phenomenon. It is also a mammal thing. As @GoodGravy mentioned, rats do laugh when they are tickled. But also primates laugh, especially when they play. Also, this is a social situation.[2, 4]
In short, laughter is a more primitive, ancient form of communication. We laugh as a social singal, not to say something is funny. Humor is not always the cause of laughter. It has to do with social situations. It is to show support and liking for the people around you. It is to make the atmosphere more charismatic and happy.
Really, the point I am trying to get is that laughter is a primitive form of communication in social situations to show support and likeness to others.
Your question is a great one and I enjoyed researching on this topic.