Each year, nearly 50 percent of Americans vow to change their behavior come Jan. 1, resolving to lose weight (a third of us want to slim down every year), get more organized or fall in love.

Odds are, they won’t succeed. Just 8 percent achieve their New Year’s resolutions. A quarter give up after the first week. These statistics are bleak but not surprising. Many New Year’s pledges involve trying to establish new habits or conquer bad ones. And there’s a lot of misinformation swirling around about how habits are formed and how they can be changed. Here are some of the most common myths.

1. A lack of willpower is to blame for our bad habits.

When people fail to change their habits, they often blame their weak wills. A third of Americans say they lack the self-control they need to accomplish their goals. About a fourth attribute trouble sticking to a diet, for example, to personal character defects such as laziness.

In truth, though, many of our behaviors are not guided by self-control. Half the tasks we perform daily are things we do without thinking. And studies show that people with high levels of self-control aren’t constantly battling temptation — they’re simply relying on good habits to exercise, make the kids’ lunch or pay the bills on time without thinking about it much. In that way, high self-control is an illusion, actually consisting of a bedrock of habitual patterns. That makes sense. It would be exhausting to repeatedly struggle to control our actions to do the right thing.

2. Apps can help us change our behavior.

Apps such as Fitbit, MyFitnessPal and BookLover promise to help us change our habits by tracking our good (or bad) behavior. And some websites say they work, running lists like “17 bad habits you can kick using nothing but a smartphone” or “Mobile apps that can help you kick your bad habits.”

But most apps simply monitor what you’re doing, which doesn’t necessarily lead to behavior change. As one group of scientists noted, “The gap between recording information and changing behavior is substantial.” There is, they wrote, “little evidence . . . that [apps] are bridging that gap.”

In my research, I’ve found that certain types of planning and monitoring actually get in the way of creating new habits, perhaps because they focus our attention on things that are irrelevant to behavior change. Some people might like these devices. But until there’s broader evidence of effectiveness, I recommend that most people don’t bother with them.

3. It takes 21 days to form a new habit.

This idea stems from a popular 1960s book by Maxwell Maltz, and it’s often repeated today. Self-help books promise that you can fix your marriage, jump-start your exercise routine or cure your money woes in just three weeks.

In truth, there’s no magic number when it comes to establishing habits. They are created slowly as people repeat behaviors in a stable context. Some simple health behaviors, such as drinking a glass of water before each meal, had to be repeated for only 18 days before people did them without thinking, according to one recent study. Others, such as exercise, needed closer to a year of repetition. Researchers found that it took an average of 66 days for a new habit to form.

For most people, more important than repeating an action for a certain number of days is establishing a routine. Doing something at the same location or time of day (like putting on sunscreen before you leave the house every morning) can help outsource control of an action. In a study of regular exercisers, for example, almost 90 percent had a location or time that cued their desire to exercise. For them, exercising was more automatic and required less thought and willpower.

4. The best way to change a habit is to set realistic goals.

In my lab, we recently conducted a study with people who wanted to change some behavior. When asked whether they would prefer a self-help book about goal-setting or one about environmental change, they overwhelmingly chose the book on goal-setting.

This is a mistake. Modifying our environment lets us remake our behavior without over-relying on willpower. Unwanted habits can be disrupted by changing the cues that activate them. For example, people eat less unhealthy food if they put lids on candy dishes at the office and if stores place unhealthy snacks at the back of displays. Altering your surroundings also can set up cues to promote desired behaviors. For example, people who weigh less keep fruit on their kitchen counters. And children without televisions in their bedrooms have lower BMIs than children with. Of course, these sorts of associations don’t prove that putting fruit on your countertop or removing TVs will make you thinner. But they illustrate how our environments cue healthy behaviors — or the reverse.

A study of returning Vietnam War veterans shows just how important environment can be. Twenty percent were actively addicted to heroin while they were serving overseas. But just 5 percent relapsed after they returned home. Researchers concluded that these shockingly low rates stemmed from the dramatic change in environment vets experienced. Back in the States, the triggering cues all but disappeared.

5. Learning about the benefits of new habits helps change our behavior.

This common misperception forms the basis for a plethora of public health efforts. For example, the federal government’s “Fruits and Veggies, More Matters” campaign has tried to educate people about the benefits of eating greens. It hasn’t worked. Since the campaign’s inception in 2007, fruit and vegetable consumption has gone down.

That’s no surprise. Research has repeatedly shown that changing knowledge and intentions does not translate into changing habits. Habits are formed through doing. And the long-term memory systems involved in habit formation don’t shift with new resolutions.

In our research, we’ve found that old habit associations endure, and hinder behavior change, even after people adopt new intentions. For example, once you see a prompt to surf the Web, it’s hard to get that out of your head and instead focus on your resolution to stay organized by paying the bills. With habits, in essence, we learn not by learning, but by doing.

Wendy Wood is a professor of psychology and business at the University of Southern California. This column was distributed by The Washington Post, where it first appeared.