No, I mean effect, i.e. causation. In this case* the direction of the correlation is 100% knowable: economic and education outcomes in the 1990s cannot possibl

No, I mean effect, i.e. causation. In this case* the direction of the correlation is 100% knowable: economic and education outcomes in the 1990s cannot possibly have altered whether or not you had a male twin in the 1970s, unless you can reverse time's arrow. *The OP was specifically asking about marriage rate and education outcomes. The paper also examined fertility. It's at least remotely plausible that there's some 3rd/confounding biological mechanism that actually the cause of both the presence of a male twin and the increased fertility of the female twin. In that case I would agree that 'correlation' is the more appropriate choice of term, but note that it's still correct in that case to assign 'effect' to both of the observed properties as they'd be downstream of the confounder.