In a previous paper it has been shown that across three cohorts of men and women born in Britain in 1946, 1958 and 1970 a gender difference exists in regard to relative rates of class mobility. For men these rates display an essential stability but for women they become more equal. The aim of the present paper is to shed light on the causes of this trend—or, that is, of increasing social fluidity—among women. We begin by considering a refined version of the perverse fluidity hypothesis: that is, one that proposes that part‐time work leads to increasing downward worklife mobility among women that also entails downward intergenerational mobility and thus promotes greater fluidity. We do in fact find that the increase in fluidity is very largely, if not entirely, confined to women who have had at least one period of part‐time work. However, a more direct test of the hypothesis is not supportive. We are then led to investigate whether it is not that part‐time working itself is the crucial factor but rather that women who subsequently work part‐time already differ from those who do not at entry into employment. We find that eventual full‐ and part‐timers do not differ in their class origins nor, in any systematic way, in their educational qualifications. But there is a marked and increasing difference in the levels of employment at which they make their labour market entry. Eventual part‐timers are more likely than eventual full‐timers to enter in working‐class positions, regardless of their class origins and qualifications. Insofar as these women are from more advantaged origins, they would appear not to seek to exploit their advantages to the same extent as do full‐timers in order to advance their own work careers. And it is, then, in the downward mobility accepted by these women—who increase in number across the cohorts—that we would locate the main source of the weakening association between class origins and destinations that is revealed among women at large.