While smoking has fallen by half among high income groups since 1970, there has been little change among the poor.

ECONOMIST: Health

"Certainly we are seeing people from very high income groups, " says Patel.

CNN: STORY HIGHLIGHTS

In addition to looking at personal characteristics such as age and relationship status, the researchers divided the countries into high- and middle-to-low income groups according to average household earnings.

CNN: People in affluent nations may be more depression-prone

The earlier group enjoyed a high degree of mobility but the later one was less fortunate, suggesting that movement between income groups is slowing down.

ECONOMIST: Class