Adolescence is a strange developmental period. The oddity of the teenage years is particularly stark to parents, who watch a major transition unfold as their children move through adolescence. That child who was focused on life with her parents — even if she played it cool by pretending not to be focused on her family — suddenly turns outward. Time by herself becomes valued over time with her family, and time with her peers becomes valued over all else. Some adolescents become suddenly convinced that their parents know nothing (and often are not afraid to make that belief known), but all teens begin experimenting with ideas that are foreign or even counter to those with which they were raised. In industrialized societies, popular culture — or running counter to it — also becomes important to the developing teen: contemporary music, movies, fashion, and sports all maintain particular attraction. And then there are the behaviors that are so vexing to parents: drinking, drugs, sex, dangerous driving, and all manner of other risky endeavors. From the parental perspective, it is easy to ask why does adolescence exist?
Biologically speaking, it is clear that adolescence is a time of profound developmental transformation. Hormonal changes trigger the growth of additional body hair and the maturation of the reproductive system, preparing the adolescent body for potential parenthood. Fat distribution and overall muscle mass both undergo major shifts, transforming the lithe, androgynous body of a child into the sexually-distinctive body of a man or a woman. While the hormonal shifts of adolescence are what we most often associate with this developmental phase, the neurological changes it brings are just as dramatic. Historically characterized as a period of psychological “storm and stress”, adolescence can now be understood through the lens of modern neuroscience. Imaging studies have shown that the brain undergoes major remodeling during adolescence [1, 2]; these changes may be the proximate cause of many of the adolescent behaviors that non-adolescents struggle to understand.
Explaining the anatomical and physiological shifts of adolescence from an evolutionary perspective is rather simple: during the teenage years, the child becomes an adult capable of having her own children. That humans take so many years to be capable of reproducing is remarkable; that such change eventually happens is not. But what about adolescent behaviors? Why is the transition to reproductive maturity accompanied by such dramatic behavioral shifts, including behaviors that have the potential to put the developing adolescent at risk of dying before reaching full adulthood? Can the behavioral characteristics of adolescence be explained from an evolutionary perspective as easily as their physiological and anatomical counterparts?
Contemporary child psychology views adolescent behavior as the product of two forms of simultaneous neurological development: “sensation seeking” and “impulse control” . While sensation seeking is thought to peak in the adolescent years, impulse control develops more slowly, leading to “maturational imbalance”  as these two processes unfold at different paces. Unable to keep up with the teen’s instinct to seek out novelty, impulse control fails to prevent behaviors that are moderated in adults, including risky behaviors. This portrayal of adolescence makes a certain kind of intuitive sense: we all know that adolescents like novel experiences, and a lot of teens have a hard time controlling impulses, some of which put them in harm’s way. But why did this imbalance between impulse control and novelty seeking evolve? Is impulse control so difficult to develop that it simply can’t keep pace with the allure of the new and different? Or have these two neurological modules — one saying “go for it”, and the other more weakly saying “maybe you shouldn’t” — evolved to develop at different paces for some adaptive reason? We shouldn’t assume that every behavior evolved in response to selective pressures, but the behavioral shifts of adolescence are so profound and potentially costly that it’s hard to imagine that they did not evolve for some adaptive reason.
The adaptive value of adolescent behavior isn’t immediately obvious, placing the “maturational imbalance” theory of adolescence on shaky evolutionary ground. Imagine an individual who develops strong impulse control just as they emerge from middle childhood. From a perspective of physical risk, this would be the ideal time for impulse control to develop: the child no longer maintains such close contact with the parent — and that parent’s judgment of what is and is not safe — and simply develops his own impulse control to avoid risk, potentially based on behavioral rules learned from the parent. Is it really possible that the requisite impulse control simply can’t develop until the young person is in her mid-twenties? And if impulse control really requires that much time to develop, wouldn’t it have been adaptive for human offspring to remain dependent “children” for even longer periods, until sufficient impulse control could be developed? If the evolved developmental pattern of adolescence puts teens at inordinate risk, it seems likely that natural selection would have long ago selected against this developmental pattern. That such a pattern exists is therefore paradoxical.
Some of the greatest biological paradoxes presented to early naturalists also revolved around risk: why would animals engage in ritual combat with other members of their species, or be adorned with conspicuous colors and anatomical ornaments, or spend so much time and energy engaging in elaborate displays? All these behaviors are risky — or at least highly costly — and therefore seem ripe for removal by natural selection. A great many of these paradoxes were resolved by considering that reproductive benefits might outweigh any costs related to survival. Understanding that sexual selection — driven by the mating environment rather than the broader ecological environment — could lead to the evolution of costly traits was a major advance in evolutionary theory. Survival is only valuable if it leads to successful reproduction, which means that risky behaviors with the potential to increase mating success have a good shot at surviving the evolutionary process.
A big part of adolescent change is sexual in nature. While middle childhood is characterized by a preference for the company of same-sex friends, adolescents shift to mixed-sex social groups and begin seeking out sexual partnerships. So it wouldn’t be entirely unreasonable to suggest that the seemingly-inexplicable behaviors of adolescence are related to mating. As the adolescent body matures, wouldn’t it make sense for adolescent behaviors to shift towards displays that might appeal to a potential mate? Perhaps the change in social orientation outside of the family, the interest in what peers value, and the desire to spend all one’s time with those peers are all about finding a suitable mate. Impulse control may be inadequately developed precisely because the resulting “maturational imbalance” leads to risky behaviors that serve as mating signals to the opposite sex. A particular level of elevated risk fostered by adolescence might be compensated for by reproductive success reaped by showing off for potential mates in one’s peer group.
The problem with viewing adolescent behavior from a purely reproductive perspective is that adolescence is not an ideal time to reproduce. Although it’s hard to determine how to compare what we know about the teenage parenthood of today with what might have applied to our ancestors, it is clear that in a diversity of contemporary societies — including many that are at the same small scale as our ancestral populations — the success rate of adolescent parents is dramatically lower than that of older parents . Being anatomically ready to be a human parent is not the same as being psychologically, culturally, and socially prepared for being a human parent. Perhaps adolescent mating displays have a long shadow, impressing potential mates that in adulthood will become actual mates. But given that maximum reproductive output occurs just as adolescence comes to an end — and characteristic adolescent behaviors subside — it seems a bit of a stretch to suggest that all of the oddity of adolescence is related to reproduction. It’s also somewhat paradoxical that adolescent risk-taking is more extreme in males, whose actual reproductive success tends to occur at a later age than females. Adolescent behaviors might serve as a kind of practice for mate competition, but are they a direct form of mating display? Probably not.
Whenever we try to account for a difficult-to-explain behavior in contemporary human societies, it is important to ask would this behavior have been difficult to explain in early human societies? Perhaps adolescent behaviors would make a whole lot more sense if we could see how they played out in our ancestral environments. Given how much humans have reshaped our environment via our rapidly-evolving culture, it is not unreasonable to suggest that behaviors that seem maladaptive in our current environment might have been adaptive in the physical and social environment experienced by our ancestors. A variety of contemporary human maladies – ranging from obesity to violent crime – have been well-explained as evolutionary mismatches: evolved characteristics that would have aided our ancestors become problematic in our dramatically-transformed contemporary cultural environment. Perhaps the paradoxical risky behaviors of adolescence result from a mismatch between current and past human environments . Our ancestors came of age in mixed groups composed of children, adolescents, and young adults, whereas today adolescent social groups are often highly-stratified by age. Perhaps some of the excesses of contemporary adolescence result from this age stratification. Or perhaps the risks available to modern teens are the problem: lower impulse control might not have been such a problem for earlier adolescents living in an environment filled with a whole lot fewer intoxicants and motor vehicles.
Explaining paradoxical traits as the result of evolutionary mismatch doesn’t just require that we show that the contemporary environment is dramatically different than the ancestral environment: the adaptive value of a trait in the ancestral environment also needs to be reasonably established. Maybe today many people become obese in our calorie-abundant environment because their ancestors survived by developing a taste for infrequently-available high-calorie foods. Maybe today some people are prone to violent crime because a willingness to engage in retributive punishment would have aided cooperation in the small social groups of our ancestors. These explanations are feasible because they explain a paradoxical trait based on both the past and present environments. The problem with explaining adolescent behaviors as a case of “evolutionary mismatch” is that it isn’t readily clear what adaptive function these behaviors would have served in the past. If adolescents used to exist in mixed-age groups – perhaps in the semi-parental role of “helper at the nest” with younger relatives – why would a preference for social company outside of the family evolve? If adolescents used to have less risky forms of display available, what function did these safer forms of display serve to developing adolescents? These questions are not unanswerable, but the lack of compelling answers calls into question the idea that the paradoxical nature of adolescence emerges from evolutionary mismatch.
All of the explanations of inexplicable adolescent behavior that we have considered thus far relate directly or indirectly to biological fitness. That’s not especially surprising given that we have been trying to explain adolescent behavior from an evolutionary perspective. But biological evolution isn’t the only driver of human behavior: we are also a highly cultural species — perhaps the only truly cultural species — and that means that we cannot discount the possibility that adolescent behavior is driven by cultural evolution. Are adolescent behaviors better understood as evolved to improve cultural fitness?
To understand adolescence from a cultural evolutionary perspective we first need to consider the nature of cultural transmission before adolescence. From birth, children depend heavily on their parents for survival. Initially this survival revolves around the provision of food and security, but eventually parents begin to provide something else: cultural education. We initially learn most of our fundamental cultural survival skills — how to obtain resources, how to make and use tools, and how to behave in social groups — from our parents. This learning occurs in early and middle childhood, periods in which contact with parents and other members of the family create the primary social environment for the developing child.
Learning from parents and other close family members is a good cultural strategy for children. As younger children are extremely vulnerable, they primarily need the protection of trusted adults. Getting that protection means spending a lot of time with members of the family, which provides plenty of opportunity to observe and imitate familial traditions: we first learn to live the cultural lives of our closest relatives. And as some forms of cultural teaching are time-consuming and therefore costly to the teacher, the child’s most reliable investors in her cultural education are close relatives. That most of our earliest culture comes from our families is not surprising.
But is mastering only the cultural traditions of one’s family the best survival strategy? What if my family’s cultural traditions are less effective than those of other families? Or what if the ecological or social environments of a child end up being dramatically different than the environments experienced by her parents? As practical and logical as it is to absorb the culture of one’s family, doing so also comes with potential risks. What if there were a way to buffer some of those risks while maintaining the potential benefits of having learned one’s familial culture?
Viewed from a cultural evolutionary perspective, the counter-intuitive behaviors of adolescence suddenly make a ton of adaptive sense. Delayed development of so-called “impulse control” might end up being a real adaptive asset: freed from the impulse to simply follow the cultural rules of her family, the developing adolescent spends a distinct period of time experimenting with cultural ideas absorbed from her peers. Think of it as broadening the cultural portfolio: the adolescent brain temporarily rejects many of her family’s ideas in order to discover, explore, and assess new ideas. Throughout adolescence, these new ideas compete with familial ideas, creating psychological turmoil and unpredictable behaviors. As adolescence subsides, the battle between ideas gained from one’s relatives and ideas gained from peers and other non-relatives also subsides. Exiting adolescence, most adults carry around a mix of cultural ideas, some from parents and some from peers. If this novel mix of ideas allows for better prospects of survival and reproduction, the cultural experimentation and resulting risk of adolescence was well worth the cost.
Adolescence has always been viewed as a process of breaking free of the family, of preparing to live independently of one’s parents. But viewed through a cultural evolutionary lens, the nature of this separation process comes into much sharper focus. What adolescence has evolved to do is to break each individual free of some or even most of the culture of the family. When it comes to future survival, it is simply too risky to rely solely on the cultural ideas passed on by one’s parents; it pays to supplement or even replace parentally-inherited culture with ideas gained from individuals from outside one’s family, most prominently one’s peers. That peer group’s developing culture, which is in the process of resolving and defining itself during each cohort’s adolescent period, is of particular importance, because the collective culture of each age cohort will become the most important social environment faced by members of that cohort. Stay too loyal to your parent’s culture as an adolescent and risk falling catastrophically out-of-step with the cultural environment you will soon experience as an adult.
Adolescence as a major human developmental phase evolved quite recently, emerging just before (or as) culture took off in the hominid lineages . This may not have been a coincidence. As our species came to rely more and more on tools as an extended phenotype and ideas as a way of fostering cooperation, the need to obtain a diverse, effective set of cultural ideas in a rapidly-evolving social environment may have given rise to a distinct developmental period in which plenty of outside ideas are allowed to compete with ideas inherited from one’s parents. Historically, psychological theories of adolescence have suggested that the development of impulse control simply couldn’t keep pace with an evolving drive towards greater sensation seeking. But what seems like a much more reasonable hypothesis is that the adolescent imbalance between increased sensation seeking and “inadequately developed” impulse control is adaptive, or at least evolved in an environment where it was adaptive.
This doesn’t mean that modern adolescents don’t face evolutionary mismatch problems. Humans at all developmental stages now have access to a breadth of cultural ideas and technologies — including ones that pose significant risk — far greater than the limited cultural palette of our ancestors. So perhaps some of the “problems” of current-day adolescence do stem from a mismatch. But the unprecedented cultural diversity that we now experience might also create the environment in which adolescent novelty-seeking behaviors are most adaptive. It’s pretty much impossible to rely solely on your parents for essential cultural skills these days, because culture is evolving so rapidly and no single person can possibly possess all the cultural ideas and skills that their offspring might fruitfully utilize in their future adult life. The healthy adolescent displays a culturally-adaptive behavioral pattern, supplementing or partially modifying the culture learned from their family. Viewed as a distinct developmental period evolved to allow individuals to improve their biological prospects by broadening their cultural portfolios, adolescence and all of its exploration, experimentation, and risk-taking make plenty of adaptive sense.
 Steinberg, L. (2008). A social neuroscience perspective on adolescent risk-taking. Developmental Review, 28, 78–106.