For the family system of Yayoi era, I do not know that yet for sure. In the Kinki region late Yayoi era, earthenware of the same pattern and production techniques have been widely distributed. From it is a case of ethnic studies creator of the world's pottery is a woman, also thought a woman had made Yayoi earthenware.
From this fact, the movement of the woman in the background with the same kind of pottery is widely distributed, ie there is also a theory that marriage can be estimated. Pit dwelling is a dwelling of Yayoi era is basically the structure and what scale of the same period, the region is almost identical. This seems to reflect the fact that lived people of similar family structure to each pit dwellings. Can you imagine from that not very large scale, and large family was not in the form to live in one of the house.
Gikokorozashi Wajin Den for the family form of Wajin, it said, "parents and siblings is a separate place to sleep." This has various interpretations, but the brothers are adults and parents is the most common interpretation that live separately.
In addition, "person of high birth 4 to 5 people, there are those with two to three wives in general Identification person" and is written, you will be considered to have been a polygamy. On the other hand, from the description of the ancient literature of Japan, such as "Manyoshu", is a theory that the ancient Japan marriage was the wife question marriage of husband and wife do not live at home is dominant. But his wife is originally, is a word that refers to a pair of men and women of one, a few while you are also written woman wife questions marriage to Manyoshu. Also, as seen in the description of the marriage of Susanoono Mikoto and comb Inada princess of "folk history", it seems there was also to build a new valley home to get married.