What does "a lack thereof" (last line) mean?

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It is a legacy of the country’s exacting domestic expectations and rigid gender roles for who performs them. While Japanese women have entered the work force at historic levels, their avalanche of domestic responsibilities is not shrinking — and men are typically not helping.

In fact, men in Japan do fewer hours of household chores and child care than in any of the world’s wealthiest nations.

According to an analysis of government data by Noriko O. Tsuya, an economics professor at Keio University in Tokyo, women who work more than 49 hours a week typically do close to 25 hours of housework a week. Their husbands do an average of less than five.

“Compared with what happened in women’s employment,” said Ms. Tsuya, “there has been so little change, or a lack thereof, in gender relations at home.”