The first and second generations used Japanese as their first language, but as the generations went by, their first language was replaced by English, the local language, and so naturally the number of Japanese Americans who do not understand Japanese has been increasing.
一世や二世の時代に第一言語として用いられた日本語も、世代が進むごとに徐々に現地語である英語にとって変わられ、自然な流れとして日本語を理解しない日系人が増えている。