Lack of exposure to "dirt" may contribute to excess cases of immune related disease in women.

The “hygiene hypothesis” links the recent rise in incidence of asthma, allergies, and autoimmune disorders such as Crohn’s disease and rheumatoid arthritis, with particular geographical and environmental locations, in particular urban, industrialized nations.  In some diseases the gender difference is 2, 3 and up to 9 to 1.  This paper discusses the possibility that the different play patterns and access to the outdoors between male and female children, may be a signficant factor.  

http://oregonstate.edu/ua/ncs/archives/2011/jan/gender-and-hygiene-could-cleanliness-be-hurting-girls

The research will be of interest to urban design professionals interested in providing natural play opportunities for children.