Financial economics is the branch of economics studying the interrelation of financial variables, such as prices, interest rates and shares, as opposed to goods and services. Financial economics concentrates on influences of real economic variables on financial ones, in contrast to pure finance. It centres on managing risk in the context of the financial markets, and the resultant economic and financial models. It essentially explores how rational investors would apply risk and return to the problem of an investment policy.
Here, the twin assumptions of rationality and market efficiency lead to modern portfolio theory (the CAPM), and to the Black–Scholes theory for option valuation; it further studies phenomena and models where these assumptions do not hold, or are extended. "Financial economics", at least formally, also considers investment under "certainty" (Fisher separation theorem, "theory of investment value", Modigliani–Miller theorem) and hence also contributes to corporate finance theory. Financial econometrics is the branch of financial economics that uses econometric techniques to parameterize the relationships suggested.