BIOTC did not come out of the blue. It was a legal "term of art" that dated back at least 200 years to Blackstone, who said parents are presumed to act in their child's best interest.But family courts today seem to mean something different. They use BIOTC to mean the opinion of putative experts who may know nothing about the wishes of the parents. But the phrase is just a smokescreen. The interests of the child are never defined, and there is certainly no analysis of what would be best. I cannot find any example of where any court actually ruled based on some legitimate BIOTC analysis. Either it assumes that the parents will act in the BIOTC, or it relies on some putative expert who is not really an expert at all.
IOW, BIOTC is just another way of expressing the principle (which also dates to Blackstone) that parents have a fundamental right to the care, custody, and companionship of their children, and to direct their child's upbringing, education and religious training.
BIOTC is not a separate principle, it is part and parcel of parental rights (a natural right or substantive due process concept) recognized by the U.S. Supreme Court in a string of cases from Meyer (1923) and Pierce (1925) through Parham (1979), Santosky (1982) and Troxel (2000).
IOW, the BIOTC is whatever the parents say it is.
Of course, a presumption can be overcome with a proper showing. Even a fundamental right can be overcome in extreme cases where a parent is found to be unfit. But unless and until a parent is found (after due process) to be unfit, a parent has the right to determine the best interest of his or her child.
I have come to the conclusion that BIOTC is one of the worst ideas in the history of human civilization. Never have I seen an idea that was so uncritically accepted among the public, and yet so utterly bogus. It is just a license for some authority figure to do whatever he wants. No good has ever come from the concept.