According to anthropologist Helen Fisher, romantic love between two partners in prehistoric times seemed to wane about years into the partnership.

Four.

Explanation:

Helen Fisher describes romantic love "as a very primitive, basic human emotion as basic as fear, anger or joy". She explained that men and women in prehistoric times who were in love were bound together in preparation for child bearing. But, after four years of raising a child through infancy, the love that once existed begins to wane. The couples tend to break up and search for new partners.

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