The social experience of adolescence is massively complex compared to that of fully-developed adults. This is evidenced both by scientific research and casual observation of social cues, like the godawful haircut I ended up with in 2007. Above all else, teenagers are desperate to fit in, battling peer pressure, social anxieties and general feelings of constant humiliation in their quest to blend. There is a reason for this, and it's because the teenage brain—particularly the structures that make up what's called the "social brain"—looks markedly different to adults' brains. As any good sixth-grade P.E. teacher will tell you, becoming a teenager comes with a whole host of physical changes; however, these changes go far beyond discovering your first pubic hair, or banging freshly-discovered hip bones into tables. Adolescence actually alters the brain's structure.
Up until approximately 15 years ago, there was a general agreement in neuroscience that most of our brain development happens in the first few years of our life. But more recently, magnetic resonance imaging (MRI) studies have shown this to be incorrect. In the teenage prefrontal cortex—the area of the brain responsible for planning, decision-making, personality expression, and moderating social behavior—grey matter is being lost, thanks to a process called "synaptic pruning." Meantime, neural pathways are boosted with a delicious fatty coating, in a process called "myelination."
In other words, brain pathways that are used infrequently are lost, and more useful connections are sped up in preparation for adulthood. It's a massive re-building that explains why social perceptions and behavioral tendencies seem to change so quickly in adolescence.
This maturation of the prefrontal cortex allows teenagers to think more abstractly outside of themselves, and look at themselves the way they feel other people are looking at them. Isabelle Rosso of the Harvard Medical School observes that this new-found ability means heightened empathy and self-regulation, but it "may also allow you allow you to have more social self-consciousness, and worry more about what other people are thinking about you. It may open up new vulnerabilities in some adolescents"
Basically, the teenage brain is obsessed with what others are thinking.
Numerous studies have found that teenagers actually use different areas of their social brain than adults when they're faced with basic emotional stimuli. Teens rely more readily on the medial prefrontal cortex, the part of the brain that processes the mental state of other people as well as mimicry (hence the copy-catting). Basically, the teenage brain is obsessed with what others are thinking.
There are a few theories around this difference: It could be because the teenage brain is designed to collect new experiences and develop social skills. Alternatively, it could be because the functions of the brain they're using require a little less oxygen, and this different way of "being" is a less demanding, more effective neural route.
Overarchingly, teens' identity and personality are also significantly altered. Tim Smith, the principal adolescent psychologist at Sydney's Psychology and Counseling Group, says this causes a social struggle in teenagers whereby fitting in takes high priority.
"Adolescence is an especially challenging time in this regard," he says, "as young people work towards establishing a more defined personal identity that is also accepting of others."
But Smith also notes that the neurological changes are just only one part of the picture. "Fascinating and complex changes occur within the developing adolescent brain that influence thoughts, emotions and behaviors, placing unique psychological challenges on young people," but these biological considerations also "sit all-too-often within challenging and demanding social settings."
Meaning that the increased demand and pressure placed on teenagers, as well as their sudden sexual awareness, contributes heavily to a heightened need for teens to fit in and run successfully with the in-crowd. Haircuts and all.