That's about laws. The "Common law" is the British legal order type. That's different from the Italian or French legal orders which are based on "Roman Laws". Thus "Common law countries" are those where Common law legal order is in use, such as UK, USA or Canada. Those countries are " known for creating maximum adversity between divorcing parents", so they are known for the fact their laws don't help divorcing parents find a peaceful agreement, but, on the contrary, their laws unfortunately drive divorcing parents to a legal conflict. "Divorcing parents" are married couples with children willing to divorce. Bye!