In investing, dual-beta is the idea that the single regular market beta can be usefully replaced with two finer-grained measures, a downside beta and an upside beta.
The Capital Asset Pricing Model posits that individual stock returns move with the overall stock market symmetrically, i.e., that their upside and downside betas are identical.
The dual-beta model attempts to differentiate downside risk (risk of loss) from upside risk (gain), both measured in terms of beta with respect to the market and not individual idiosyncratic risk.
In practice, it is difficult to estimate the future downside market-beta.
Indeed, the historical single beta is a better predictor of the future downside beta than the historical downside beta.