The Secret to Finding (and being) a Good Mother-in-Law

Relationships with mothers-in-law are famous for being rocky,if not impossible. My relationship with my own MIL is turbulent,at best. I've long been interested in why this is so, as I'm fascinated with people and why they do the things they do.
When my sisters began dating seriously and would complain about their boyfriends' moms, I became even more interested. Our homelife was a little odd and we all came out of there with trust issues and sometimes with chips on our shoulders. I often wondered if our dealings with the moms of our men were so unpleasant because the women themselves really were unmanageable,or if it were something we brought on ourselves due to our suspiciousness and lack of social skills in those areas.
Then one of my sisters became involved with a guy who has a mother she is absolutely smitten with. My sister does lots of things with this woman because she actually wants to-not out of any sense of obligation. Of course this sent my curiousity into overdrive. I now had a sort of experimental in the control group that was my other three sisters and myself. What factor existed that caused the one sister to actually find a mom of a love interest tolerable? She had the same background as the other three of us, so I could theoretically study my sister and figure out if the problem in MIL sorts of relationships was Us or Them!
After listening closely to my sisters when they complained and also analyzing the thorns my own MIL had ground into my side, combined with studying the things my sister enjoyed about her boyfriend's mom, I had my conclusion: The good mom of the boyfriend has DAUGHTERS whereas all of the less pleasant moms only have sons!

It made perfect sense! The daughterless moms had no real sense of boundaries. For instance,they would assume a deep friendship that hadn't had time to develop. This caused them to be inappropriately heavy with the TMI and pushy in wanting to spend time together. They also wanted to gush a bunch of phony I -love-you's practically after the first date. Plus, they would get snappy and try to give instructions on how to best take care of their "babies". Also,not to sound materialistic,but the sons-only moms would give the strangest gifts (used lingerie,anyone?.

Since the more agreeable mother had a daughter built right into her family, she didn't have a need to force the mother/daughter relationship. She could curb her enthusiasm,if there even was any,and just be cool. A bonus: she gives exceptional gifts-she knows what girls like. After further analysis,it dawned on me that one sister even has a step-MIL that she adores,who has the same qualities and gift-giving skills. Guess what? This step-mom has a daughter! Further proof! Of course some people are simply more pleasant than others but I still think I'm on to something.
Now,to my readers who have sons only,keep this in mind and try not to be creepy when they start dating. And to you single ladies: Only date guys with sisters! You can all thank me later! :)