From Fooducate

A study just published in the New England Journal of Medicine has found a correlation between several bad habits and weight gain:
Within each 4-year period, participants gained an average of 3.35 lb (5th to 95th percentile, −4.1 to 12.4). On the basis of increased daily servings of individual dietary components, 4-year weight change was most strongly associated with the intake of

• potato chips (1.69 lb),
• potatoes (1.28 lb),
• sugar-sweetened beverages (1.00 lb),
• unprocessed red meats (0.95 lb), and
• processed meats (0.93 lb)

There was an inverse association linked to the intake of

• vegetables (−0.22 lb),
• whole grains (−0.37 lb),
• fruits (−0.49 lb),
• nuts (−0.57 lb), and
• yogurt (−0.82 lb)

What you need to know:
What do these numbers mean? One could argue that eating potato chips means you’ll get fat. But it’s hard to prove causality in these types of studies. Perhaps the people eating chips and soda were less health conscious than those eating yogurt and veggies.
This brings up the issue of calorie quality vs. calorie quantity:

Finally, it’s interesting to note that potatoes (not just potato chips) are associated with weight gain. While potatoes in and of themselves are a healthy food, their main use in the American diet is as french fries.