An interesting piece of research has been reported on by the BBC today.
The research looked at the relationship between inactivity and obesity. Most previous research has either assumed or shown the inactivity leads to obesity.
In contrast, this new research suggests that the direction of the relationship is the other way around - obesity leads to inactivity.
This is a nice example of the "correlation does not imply causation" mantra and presents a nice chicken and egg situation.
The original study can be found here.