The question: Experts have long known that heart attack rates, for reasons still unclear, shoot up during winter time, and previous studies have suggested that colder temperatures may be to blame. "Winter can be deadly," says Charlene Laino at WebMD, "at least when it comes to matters of the heart." Is the cold really to blame?

How it was tested: Dr. Ryan Schwartz of Good Samaritan Hospital in Los Angeles analyzed more than 1.5 million death certificates from 2005 to 2008. His team studied three warm states (Texas, Arizona, and Georgia), two areas with moderate climates (Los Angeles County and the western half of Washington State), and two states with cold winters (Massachusetts and Pennsylvania) to determine whether colder climates led to more deaths.

The outcome: The raw data showed that the number of deaths unsurprisingly spiked during winter as compared to summer — rates were 26 to 36 percent higher. More surprisingly, though, Schwartz' team found no statistical difference between the locales with harsher winters and those with mild winter weather. In other words: Cold temperatures weren't to blame.

Why this is: It could be all the other factors associated with winter time that makes people more vulnerable to heart attack, such as higher rates of flu infection, less-healthy eating, a lack of exercise, and higher rates of depression. In all likelihood it's a combination of all these factors. For example, says WebMD's Laino, "a person who is on cholesterol-lowering and high blood pressure medication might feel down, making them less likely to take their medication and more likely to reach for calorie- and fat-packed snacks."

What experts say: The results are surprising, said Dr. Bryan Schwartz in a statement, because "climate was thought to be the primary determinant of season variation in death rates." Other experts agree: "I'm in the Chicago area," says the American Heart Association Dr. Vincent Bufalino, "and come winter, we have to prod our patients to go to the gym, eat right, and keep it up." Or maybe, says researcher Dr. Robert Kloner, "it's obnoxious relatives, or financial stress."