A culture is described as having polygynous marriages and patrilineal descent but data shows that fewer than twenty percent of the population is in a polygynous marriage. This illustrates what dichotomy

ideal/real dichotomy

Explanation:

A dichotomy is a partition into two mutually exclusive or contradictory groups. That is, a division into groups A and B where A and B do not overlap.

In this case, the real data shows that this population is not mainly polygynous. On the other hand, the ideal representation of this culture is that of a polygynous one.

Therefore, this represents the ideal/real dichotomy.