Imagine there is a marshmallow sitting on a plate in front of you. A nice lady in a lab coat sits next to you. She tells you she's going to leave the room, and you can eat the marshmallow if you want. But, if you wait for her to get back, she'll give you two marshmallows. Oh and by the way, you're only four-years old in this scenario. So which is it: one marshmallow now, or two later? Choose wisely. It will impact the rest of your life.
The famous marshmallow experiment was conducted 40 years ago. Kids who waited the longest grew up to be more successful than the kids who ate the marshmallow right away. They got better grades in school, got more education, and ended up making more money. The length of time waiting for the marshmallow was a greater predictor of success than IQ.
So what does this experiment tell us about the brain? Behaviorally, it is a test of willpower: the ability to control yourself and your actions. Neurobiologically, it is essentially a battle between two neurotransmitter systems: dopamine and serotonin. These are two similarly structured chemicals known as monoamines. Despite their similarity, they have very different roles. Dopamine is responsible for the pleasurable, rewarding feeling you get from eating food, having sex, winning at roulette, doing drugs, and a million other fun things. It acts on a deep area of the brain known as the basal ganglia. As soon as you see something potentially rewarding, dopamine compels you to pursue it. In contrast, the serotonin system is particularly important in mediating activity in the prefrontal cortex. Specifically it helps you to override your immediate impulse to just eat the marshmallow in front of you (check out this entertaining video of kids in this experiment).
The marshmallow experiment illuminates an important essence of who you are. How your brain developed early in life, forming the relative strength of these two neurotransmitter systems, will shape the course of the rest of your life. Despite it's apparent simplicity, if you think about it, the proposition in the experiment is quite strange: one marshmallow now, or two marshmallows later. It seems straightforward, but in fact there aren't two marshmallows. I don't mean the experimenter is lying, just that there is actually one marshmallow sitting in front of you in all its white, puffy, delectable sweetness. The two marshmallows do not presently exist in front of you; they're only an abstraction of your mind. So ultimately the marshmallow experiment is a test of your brain's ability to choose between something real and tangible and present, or something abstract that exists only in your mind.
The powerful results of this experiment, therefore, demonstrate how much your choices depend on the abstract notion of what is meaningful to you versus what exciting thing is currently available to you. The former depends on the serotonin system, while the latter depends on the dopamine system.
Sidebar: when I say "serotonin system" or "dopamine system" I'm not only referring to the neurons that create that particular neurotransmitter and squirt it into synapses all over the brain, but I'm also including the receptors on adjacent neurons. It also includes the molecules that suck the neurotransmitter back into the cell, and the processes that break it down. Therefore the phrase "increased serotonin activity" can mean a number of different things. For example, it can mean increased production of serotonin, or increased number of receptors for serotonin, or decreased re-uptake of serotonin, or decreased degradation of serotonin. Now back to our regularly scheduled program.
To give you some direct evidence of these systems at work, I'm going to describe a 2005 experiment from Oxford. In this experiment the scientists manipulated serotonin or dopamine independently, by injecting chemicals that block serotonin or dopamine activity respectively. Don't worry; these experiments were done on rats not kindergartners (I'm guessing the scientists would rather face PETA than the PTA). The basic setup consisted of a rat in the middle of a hallway with 2 food pellets at one end and 4 at the other. Now rats aren't necessarily the brightest, but they, just like you, will always supersize it as long as it's free. Even if serotonin or dopamine transmission was mostly blocked, they still had the sense to go for the bigger reward.
Then the scientists put up a wire fence in front of the 4 pellets. Rats are good climbers, and the regular rats, with all their neurotransmitters present, were happy to climb over the fence. However, the dopamine-blocked rats looked at the wire fence and thought, "It's not worth it," and went for the 2 pellets (Note: I'm anthropomorphizing the rats, this wasn't The Secret of NIMH or anything like that). In contrast, the serotonin-blocked rats still eagerly clambered over the fence for the big reward. With the big reward in plain sight, their fully functioning dopamine system took over, and they greedily went for the big pile of food.
Now if instead of a barrier the scientists made the rats wait 15 seconds for the big pile, then things got a little more interesting. Regular rats were happy to wait. Dopamine-blocked rats again went for the two easy pellets sitting right in front of them. But in this case, the serotonin-blocked rats also went for the two pellets. They were unwilling to wait. Basically the rats looked over at the high reward side, and were like "You mean I really have to wait 15 seconds? Screw that." And they wouldn't wait even if the reward was increased to 10 pellets. During the "fence" iteration they were willing to work hard, but that was simply because they could see the reward, and the dopamine system took over. Without the tangible reward in sight, they were unable to control the dopamine impulse to just eat the 2 pellets in front of them. Serotonin is necessary, though not necessarily sufficient, to drive the pursuit of abstract, future goals.
Now this could have a huge impact on your success and happiness in life. If the cute young blonde at the gym asks you on a date, will you act on impulse and attraction, or instead act on some abstract notion of being faithful to your wife? Freshman year of college, are you able to resist the lure of the party down the hall in order to study, simply because someday you want to be a doctor? The serotonin system allows you to make the choices you want, rather than have the choices be made for you by circumstance. In short, proper serotonin activity is necessary to avoid poor life decisions.
Low-serotonin activity can lead to impulsivity, aggression, addictive behaviors, and depression. Of course, since you have "free will" (let's not argue about this now), you can always make the "right" decision, even if it is the difficult one; it's just that serotonin makes it feel more possible. Essentially that is what antidepressant medications do: they don't make you happier directly; they just boost serotonin, which increases your willpower.
On a personal note, writing these posts is enjoyable and fulfilling, but sitting by myself for hours on end in front of a computer certainly is a test of my serotonin system. In the next post I'll explore natural methods of increasing serotonin activity. Look for it soon, unless of course I get sidetracked by a marshmallow.
If you liked this article check out my book - The Upward Spiral: Using Neuroscience to Reverse the Course of Depression, One Small Change at a Time