The case against polygamy

Via Science Daily "Two People Can Learn to Cooperate Intuitively, but Larger Groups Need to Communicate, Research Shows"

In the game of life, while two people may 'develop an understanding' or work intuitively together -- this scenario is easily distorted once a third person becomes involved. Without effective planning and ground rules, even the best of working relationships between two people can become undone once a third is involved.

"Married couples or pairs of business partners may be able to rely on this type of intuitive cooperation, to an extent, but larger groups need explicit communication and planning. Mechanisms need to be put in place to facilitate it. Intuitive cooperation is really a case of two's company, but three's a crowd."

The example in the study were not married couples.