I don't have research, but I have my life experience. My father was Jean-Pierre Abbat (you can look him up on Wikipedia). Being French and an engineer, he thought in metric. I had lots of French books growing up, and started learning French no later than 3, and I still speak it. If I went to France, they'd probably think I'm from Normandy (I definitely do not sound Parisian). I also learned Spanish from my mother, and understand it fairly well, but do not speak it fluently. (My father never learned Spanish, but my mother speaks French.)
My father also had various measuring tools: tape measures, durometer, etc. At an early age I learned that things are made in round metric sizes: a Lego block is 8 mm wide. (It is 9.6 mm tall, but I didn't find that out until a few months ago. The measuring tape didn't have a resolution that fine. I also learned binary early, so 96 is a round number to me.) Having been in America for many years before I was born, he had a thermocouple reading in Fahrenheit (to measure the grill), but I already thought that was odd. When I took physics in high school and we measured ten meters in the classroom, hung kilograms on the balance, and measured their weight in newtons, I went completely metric. Several years ago I went to Portugal to help construction of a church building. I was the only one of 30 who brought a metric tape measure. phma
