The joke is that common sense isn't very common. But what is it?
Pew Research has studied the effect of religion on every day life and decisions, and arranged it by state. The percent of adults in Ohio who say they look to religion most for guidance on right and wrong is only 33% and 47% say they look to common sense. However, in 2007 it was 27% look to religion and 57% to common sense. Maybe the recession sent a few of them back to church. Only 32% of Ohioans say there are clear standards of right and wrong, whereas 66% says it depends on the situation.
For my generation "common sense" probably was the values and ethics handed down by parents and grandparents, which they most likely learned in church. My parents were born in 1912 and 1913, they both grew up on farms, attended one room schools, and neither one had any choice about going to church, and in those days that meant worship with adults. They heard about how treat their neighbor and the poor, they knew from observation and lectures about an honest day's work, they were told the wages of sin is death. My siblings and I heard a similar message either in church, at school, or from our parents and it was reinforced in club activities like Girl Scouts and 4-H or church choir practice and in our friends' homes.