I like my shopping routine at the grocery store around the corner, where my cart seems to easily navigate itself through the isles. Once in a while I make adventurous purchases (the Halloween-edition beer with pumpkin aroma still awaits in my fridge), but I usually stick to the products that have already made me happy before. Whenever in a new town, I try to shop at the same chain, where I know the products and their location on the shelves.

One morning, the parking lot of the usual store was packed. I decided to explore and went to the supermarket across the street for the first time.  My experience at the new place was mildly traumatizing: confronted by all sorts of new packages calling from the shelves, I felt helpless. How should I know what I would like?

In my first blog post I wrote about our mind’s limited computational capacity. When we shop, examining all of the products, predicting how much we will enjoy them and whether the price is reasonable (compared with the alternative) is very costly in terms of cognitive effort. Luckily, our brains have figured out a shortcut that saves a lot of time and mental effort, making it easy to navigate through the local convenient store and fill ours cart with the usual products.

Faced with a novel situation, we usually don’t know what to do. Sooner or later, after a short exploration period of trial and error, we learn what works for us. Animals are not much different: in 1898 American psychologist Edward L Thorndike invented the “puzzle boxes” – cages that the animal (say, a cat) could exit (and get a food reward) only by using a specific response (like pulling a lever, for example). Thorndike found that the time it took the animals to escape and get their food reward decreased with their experience.

This is not a news for anyone who ever had a pet: animals quickly figure out what action would get them the reward. The process of associating a state of the world with an action and its outcome in the anima’s brain, called “instrumental conditioning”, can be used to teach animals, like raccoons, to perform complicated tasks, like playing basketball.

Do animal learn to associate the sensory stimulus and their responsive actions with the reward, or maybe they are just reinforced to perform the action, regardless of the reward value? To answer this question, Dickinson and colleagues (1995) trained mice to push a lever in order to get food. After 120 successful trials, the mice were fed to satiety. Lacking the motivation to get food, the mice stopped pressing the lever, providing evidence that they had learnt the consequence of their actions, and weren’t just blindly responding to the lever in their cage.

Dickinson repeated his experiment with a new group of mice, but this time he extended the training period to 360 (rather than 120) trials. The effect of over-training on the mice behavior was dramatic: even after they were fed to satiety and no longer wanted the food, they kept pressing the lever. The mice picked up a lever-pressing habit.

Although we all have natural, automatic responses to events in our environment (like the urge to step out of the elevator even though it had stopped in the wrong floor), us, humans, have a sense of control. We believe in our ability to suppress habitual tendencies more easily than other animals. In 2009, Elizabeth Tricomi and colleagues recruited Rutgers college students, making sure that none of the subjects were dieting. Participants fasted for six hours, and then performed a simple task: in each trial, they were instructed to push one of four buttons in order to get a food reward. In some trials, the reward was an M&M; in others it was corn chips (the researcher verified that all of the subjects liked both foods).

Like in Dickinson’s mice studies, subjects were divided into two groups: the first performed the task only for a single day, in two 8-minute training sessions, where the second performed four sessions each day for 3 days – six times as much training. Now came the fun part: after the last training session, participants were given one of the food types, and were instructed to eat until it was no longer pleasant to them. With one of the food “devalued”, subjects went back to perform the task for three more minutes. What do you think happened? When the rewarded food was no longer desirable, the first group stopped pushing buttons, as expected. Surprisingly (or not) the over-trained subjects became habitual, and kept pressing buttons even when the food reward had become unpleasant.

Habit formation imposes a trade-off. Most of the time, the shortcut works well: we easily navigate our cars and carts in familiar avenues without having to pay too much attention. Faced with a new situation, however, we are prone to make mistakes, or even worse – pick up unhealthy habits. Obesity, alcoholism and other addictions are often a result of a long reinforcement history (of alcohol, drugs or fried-chicken) that ended up turning into out of control habits.

Accepting the habitual system as an inseparable part of our minds, understanding its limitation and the way it works, may help us to achieve our long-term goals. Forcing ourselves to start working out and eat healthy is effortful, but the investment will pay off. After a period of reinforcement (by endorphin rush, feeling lighter and getting compliments), our brains will pick up the habit. Going to the gym or having kale salad may become effortless or even enjoyable.