Academic research on social mobility from the 1960s until now has made several facts clear. First, and most important, it is better to ask how the conditions and circumstances of early life constrain adult success than to ask who is moving up and who is not. The focus on origins keeps the substantive issues of opportunity and fairness in focus, while the mobility question leads to confusing side issues. Second, mobility is intrinsically symmetrical; each upward move is offset by a downward move in the absence of growth, expansion, or immigration. Third, social origins are not a single dimension of inequality that can be paired with the outcome of interest (without significant excluded variable bias); they are a comprehensive set of conditions describing the circumstances of youth. Fourth, the constraints of social origins vary by time, place, and subpopulation. These four “knowns” should inform any attempt to collect new data on mobility.