For men, relationship happiness was directly proportional to the length of a relationship.

对男性来说，交往愉悦程度直接与交往的长短成正比。

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The relationship between two quantities when a change in a second quantity is directly proportional to a change in the first quantity.

两个变量之间的一种关系，第二个量的变化直接和第一个量的变化成比例。

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