Latest figures from RAC Legal Services road safety study show the influence of parents is almost twice as powerful as that of Government education programmes on improving driver attitudes.
Drivers said they were less likely to suffer from road rage, exceed speed limits or be tempted into 'risky' manoeuvres on the road with family in the back seat. In total, 31% drive more carefully with their children, while one in four (25%) calm down with their mother-in-law in the car.
RAC Legal Services spokesman, Philip Hale, says: "For many drivers, it seems their mother-in-law is one step away from traffic police when it comes to improving attitudes behind the wheel. Driving the family is less likely to feature the pressure of a work-related journey and can make us more responsible, careful and self-conscious."
The research also shows the potential danger of peer pressure, with 30% of men saying they would drive differently in front of their mates - something much less of an influence amongst women (21%).
The greatest positive impact on driving behaviour is actually from outside the car, with seeing a police car having the most profound effect (53%), ahead of clearer signs or road markings to highlight dangers (34%).
Hale continued: "We can see when we're more likely to take risks or be tempted into bad habits, but we can also see what influences drivers positively. RAC would like to see more traffic police on the roads and a wider use of interactive signs, to better highlight danger points to drivers - although we'd stop short of suggesting everyone drives with their mother-in-law."