If those subway ads are more effective against the cautious Martins than against the reckless Maxwells, then they are a threat to the hapless Joans. The Maxwells are more cautious than the Martins are.
If those subway ads are more effective against the cautious Martins than against the reckless them,then they are a threat to the hapless Joans They are more cautious than the Martins are.