van Bekkum, S., Pennings, E. and Smit, H. (2009). A Real Options Perspective on R&D Portfolio Diversification Research Policy, 38(7):1150--1158.

This paper shows that the conditionality of investment decisions in R\&D has a critical impact on portfolio risk, and implies that traditional diversification strategies should be reevaluated when a portfolio is constructed. Real option theory argues that research projects have conditional or option-like risk and return properties, and are different from unconditional projects. Although the risk of a portfolio always depends on the correlation between projects, a portfolio of conditional R\&D projects with real option characteristics has a fundamentally different risk than a portfolio of unconditional projects. When conditional R\&D projects are negatively correlated, diversification only slightly reduces portfolio risk. When projects are positively correlated, however, diversification proves more effective than conventional tools predict.