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From The Oxford Book of Modern Science Writings by Richard Dawkins.

He introduced the 'unsentimental calculus' of parental investment theory, and nowhere was it more unsentimental than in Triver's theory of parent offspring conflict. Trivers thinks about evolutionary strategy in economic terms. His concept of parental investment is defined as an 'opportunity cost': the amount invested by a parent in any one offspring is measured as the lost opportunity to invest in other offspring.