(4) This inferior efficiency is not solely attributable to these reasons. Woman's incentive to acquire industrial efficiency is not so great as man's. A large number of women-workers do not enter an industrial occupation as the chief means of support throughout their life. The influence of matrimony and domestic life operates in various ways upon women's industry. The expectation of marriage and a release from industrial work must lessen the interest of women in their work. The fact that even while unmarried a large proportion of women-workers are not dependent upon their earnings for a livelihood will have the same result. A larger proportion of the woman's industrial career is occupied in acquiring the experience which makes her a valuable worker, and the probability that, after she has acquired it, she may not need to use it, diminishes both directly and indirectly the net value of her industrial life; the element of uncertainty and instability prevents the advancement of competent women to posts where fixity of tenure is an important factor. Where married women are engaged in industrial work either in factories or at home, domestic work of necessity engages some of their strength and interest, and is liable to trench upon the energy which otherwise might go into industry. Even unmarried women have frequently some domestic work to do which is added to their industrial work. Thus the incentive to efficiency is weaker in woman, her industrial position is less stable and her industrial life shorter, while part of her energy is diverted to other than industrial channels. (5) There is conclusive evidence to show that women are more often absent from work owing to sickness and other claims upon their time than men.[250] Though closely related to the former factors this may be treated separately in assessing the net productiveness of women, because it is distinctly measurable. But in touching this point it should be remarked that weaker muscular development does not necessarily imply more sickness. The loss of working time sustained by women could probably be reduced considerably by more attention to physical training and exercise and by a higher standard of diet. (6) Although the limitations of law and custom, which limit the hours of labour for women in many of their industrial occupations and forbid them to undertake night-work, cannot be reasonably held to reduce the net efficiency of women's labour taken as an aggregate, they must be allowed to diminish the direct net productiveness of women in certain employments as compared with men, and either to bar them out of these employments or engage them upon lower wages. In certain textile factories where goods of some special pattern are woven at short notice, and where overtime is essential, women cannot be employed. In the Post Office, where night-work is required at certain seasons, women are at a disadvantage, which is doubtless reflected in the lower wages they receive.