The Equally Yoked Dyad versus an Unequally Yoked Pair

What's the difference in Equally Yoked vs unequally yoked? What has science revealed? Any country/any culture? The studies emanating from the scientific communities of the U.K. and Canada often have differing of opinions----So does Australia and the U.S.A. in Dyadic Psychology. Question: What, now, is the world summit leaders and the mainstream 'consensus' amongst the brightest of social scientists assert?