Science keeps on coming up with very persuasive reasons to quit smoking — according to recent studies, cigarettes boost the odds of dementia, depression, and bad cholesterol, while quitting them makes you happier. Now, science is also shedding light on why millions of Americans fail at their efforts to kick the habit each year. Here's a look at five studies that might explain why so many are still lighting up.

1. Certain smokers are hardwired to smoke
Researchers at the University of Michigan say that brain scans can predict whether a smoker will quit or not. In their study, 28 trying-to-quit heavy smokers were hooked up to a functional MRI scanner while they watched TV ads aimed at helping smokers kick the habit. Those participants whose brains showed activity in the medial prefrontal cortex during the ads were doing much better at quitting a month later. "It seems that our brain activity may provide information that introspection does not," says researcher Emily Falk.

2. Night owl? Good luck quitting
Finnish researchers found that self-identified "evening types" are three times as likely to be smokers as "morning types," and significantly less likely to quit. The study is based on a 30-year look at 23,000 pairs of twins. Nicotine, a stimulant, may simply keep people up at night, says Ulla Broms at the University of Helsinki, but another explanation may be that people whose brains have active opioid and dopamine systems, tied to pleasure and addiction, are more likely to both smoke and stay up late, she suggests.

3. Quitters, and doctors, are too impatient
Some people quit smoking right away and stay on the wagon, but others take more time and a few lapses, says a study from Oregon Health Science University. Unfortunately, smokers in this second category, and their doctors, often give up on a cessation strategy before it's had a chance to work. The smokers in the Oregon study were taking one of two cessation drugs or going to counseling, and a third of the successful quitters fell off the wagon at least once in the first 12 weeks.

4. Some smokers just live in the wrong state
If you're having trouble quitting, consider moving to a less smoker-friendly area, suggests a study from Harvard Medical School and the Massachusetts Tobacco Cessation and Prevention Program. When the Bay State started offering free anti-smoking drugs, patches, nicotine gum, and other treatments to Medicaid participants, 40 percent of Medicaid smokers said yes, and smoking dropped 10 percent. Heart attacks and heart disease also both dropped by almost 50 percent. Only five other states offer comparable programs, while another 20 or so offer partial insurance coverage for smoking cessation treatments. The researchers suggest that smoking bans help, too.

5. Anxiety disorders encourage smoking
People with anxiety disorders are more likely to smoke, and find it harder to quit, say University of Wisconsin researchers. They also respond poorly to common treatments like nicotine lozenges and patches, and experience harsher withdrawal symptoms, the study found. Perhaps not surprisingly, people with histories of panic attacks, social anxiety, and general anxiety were more jittery about quitting, even if they weren't heavy smokers. These smokers might do better with intensive counseling than anti-smoking drugs, the researchers suggest.