Social groups in male and female prisons in the United States differ in the social structures and cultural norms observed in men's and women's prison populations. While there are many underlying similarities between the two sets of populations, sociologists have historically noted different formal and informal social structures within inmate populations. One of the most common and prominently noted differences is the appearance in women's prisons of pseudo-families, which, while they have been discussed in sociological texts since the 1930s, have not been noted in men's prisons at any point. This difference is a manifestation of gendered social factors which influence male and female populations within prisons and within larger society. Population differences The populations and demographics of male and female inmate populations in the US are very different from one another. These differences factor heavily into theories on prisoner social organization. The most significant difference is in the relative size of male and female populations; in 2014 there were approximately 1,440,000 male and 112,000 female prisoners in the United States. The much larger size of the male prison population causes major differences in the institutions in which male and female inmates serve their time. Another significant factor in prisoner social structure is the differences in arrest factors for male and female prison populations; higher proportion of female inmates are incarcerated due to a property or drug charge than are their male counterparts, a higher proportion of whom have been arrested for a violent offence. Female inmates are more likely to have been the sole provider for their children prior to their incarceration; 46% of fathers in prison self reported to have been living with their children prior to arrest, this number was 64% for mothers. Additionally, because there are fewer women's prisons, incarcerated mothers tend to receive fewer visits from family than incarcerated fathers typically do as they are often living in a facility which is hundreds of miles from where their children are living. Incarcerated women tend to have significantly higher rates of mental health problems than male prisoners; 73% of women in state prisons suffer from some mental health issue versus 55% of men. Inmate social groups and culture The societies and cultures within prisons frequently have been of interest to sociologists; one early study, published in 1949, noted the significance of prison community and the structure of the inmates' society. The author, F.E. Haynes, found that