If you think about your brain at all, you probably think of it as a well-developed organ that, by the time you're in your 20s at least, has matured into whatever it will be for the rest of your life. But that's not true at all. In his new book, Behave: The Biology of Humans at our Best and Worst, Stanford professor and TED speaker Robert Sapolsky explains the complex interplay of brain biology and social conditioning that prompts both good and bad human behaviors, and how both our brains and the behaviors they prompt can and do change, for better or worse, throughout our lives.

Imagine that you're a neurosurgeon. Years of practicing your profession will expand the part of your motor cortex devoted to finger dexterity. That's good for your career, your hospital, your patients, and society at large. But if you're a safecracker, years of working at that profession will have the same effect on your motor skills--and that's not so good for society, and maybe not even for you in the long run.

Driving a taxi in the famously complex and confusing streets of London will actually expand your hippocampus, which controls your memory. On the other hand, severe trauma causes post-traumatic stress disorder (PTSD), in which your hippocampus atrophies, and your amygdala, your emotional center, expands, making you more prey to your feelings and stressors and less able to think rationally.

What can we learn from all this?

1. Humans are better--or at least more capable of being better--than you might think.

Take altruism, for example. You may think that's an innate quality in people (or at least some people), but Sapolsky points out that there's no evolutionary explanation for it. From a Darwinian perspective, you're more likely to survive and thus better off running away from another being in distress than lending a helping hand. So why do humans (and other creatures) so often help each other instead? It's social behavior, often learned at such an early age that it's deeply ingrained, Sapolsky says. And it's a good reason to be at least somewhat optimistic that we're evolving away from our worst behaviors and toward better ones.

2. To understand why we do what we do, you have to go back in time.

Imagine you're a police officer in the middle of a chaotic situation who sees someone running toward you holding something. You have less than a second to decide what to do and you believe the person has a gun, so you shoot. You then realize that what you thought was a gun was actually a mobile phone.

To understand why you made the mistake and fired on an unarmed person requires going back to a few seconds before the event. Were you in the grip of your emotions (that is, your amygdala)? Then go back to a few hours or days earlier. Was your testosterone level high? (Both women and men experience testosterone changes that can influence behavior, although men have much more testosterone than women do.)

And that's just the beginning. To fully understand your own behavior, you would need to understand what influences were affecting your brain during adolescence, a time when your experiences and environment can greatly influence your brain's development. So can prenatal experiences and hormones, for that matter. And then there's the way our ancestors' brains developed through the millennia. All these things affect what we do and the conscious and unconscious choices we make throughout our lives.

3. Stress is even worse than you thought.

PTSD is only one example of how a stressor (severe, in this case) can affect your brain. There are countless others. In our example of the police officer who mistakes a phone for a gun, prolonged stress in your work or home life could cause your amygdala to grow larger, making it more likely to dictate your behavior.

Sapolsky has also noted there's evidence that chronic stress causes DNA to age faster--which basically will make you age faster as well. What's interesting about humans (and other primates), he notes, is that we tend to be stressed by things that aren't really immediate threats. Stress response evolved to cause the body to devote as many resources as possible to activities that would allow us to escape, such as running faster. To do that, it pulls resources away from pretty much everything else.

Thus, stress winds up shutting down our digestive system, cell production that would heal wounds, and even, over time, our reproductive organs. If we experience stress every day, due to a bad boss or trouble marriage, for instance, we can experience all those deadly effects over time.

But some people handle stress better than others, and neuroscience is working to figure out why. In the meantime, Sapolsky advises, make it a priority to cope with the stresses in your own life through tools like stress management, meditation, or even therapy. Because stress is even more of a killer than you thought.