Ask the average person what causes obesity, and you'll likely get one of these basic answers (or some version thereof):1. "Eating too much!"2. "Not exercising enough!"3. "Genes!"OK, sure. Seems logical. But while each of those answers may be accurate in its own way, there are other, more surprising, factors that have been linked to obesity, too. No one factor is responsible for obesity, and it's often difficult to prove a direct cause-and-effect relationship. But these sneaky-like-a-thief things might be stealing your family's good health:Is stress making you gain weight?Air pollution. It doesn't just hurt your lungs: A 2012 study found that prenatal exposure to high levels of polycyclic aromatic hydrocarbons (PAHs) in the air made children twice as likely to be obese by age 7.The takeaway? If you're pregnant, steer clear of car exhaust and smoke from cigarettes, fireplaces and barbecue grills as much as possible. For more info about PAHs, click here.Altitude. The closer you live to sea level, the heavier you're likely to be, found a study published in the International Journal of Obesity. In fact, people living at elevations below 1,640 feet were 5.1 times more likely to be obese than people living above 9,843 feet. Researchers controlled for things like diet, fitness, temperature, etc.; they think the "increased metabolic demands" of living at high elevations may be behind the phenomenon.The takeaway? If you're livin' low, there's not much you can do to change your altitude (short of moving to the mountains), but... why not take a ski vacation? Couldn't hurt!Tips for family fun on the slopesAntibiotics. According to a 2012 study, there's a correlation between obesity and early use of antibiotics in children. A study on mice resulted in similar findings. Researchers believe it might be due to how antibiotics change gut bacteria (more on that below).The takeaway? Talk with a doctor before giving children antibiotics -- or even taking them yourself. Discuss ways you can normalize your gut bacteria while on antibiotic therapy.BPA. A 2013 study found that children with high levels of BPA (a chemical found in lots of plastic products) had increased odds of obesity -- and earlier research has found the same to be true for adults.The takeaway? Easy: Limit your exposure to items that contain BPA -- and that means not even touching them, since BPA is easily absorbed through the skin. For a list of common items that contain BPA, click here -- and beware of buying bottles and sippy cups at garage sales and consignment stores, since they may predate the FDA's 2012 ban of BPA in those products.upwave: Should you replace your kids' dishes?Car dependence. As one 2012 study points out, driving is sedentary behavior -- and the more sedentary you are, the greater your obesity risk.The takeaway? If your destination's five blocks away or closer, walk. Even if your daily commute is long, you can walk partway to work, too.Fast-food salads. Salads from several food chains have been found to contain 1,200 calories or more -- including some that contain sounds-good-for-you grilled chicken. As a result, customers consume way more calories than they'd bargained for, and more calories equals potential weight gain.The takeaway? Get over the idea that the word "salad" automatically means "healthy." For tips on how to slim down your salads, click here.Eat your way to a faster metabolismGut bacteria. Research has found that people whose intestines lack "microbial diversity" have a higher risk of weight gain. Translation: The more kinds of bacteria you have in your guts, the lower your risk of obesity.The takeaway? Read up on probiotics, and think of ways you can work them into your regular diet. (Kefir, anyone?)Lack of sleep. According to a 2013 study published in Nature, sleep deprivation both motivates us to eat and blunts our ability to make good food choices -- a combination that can contribute to obesity.(CNN)ANN.Az