There were an estimated 550,000 homeless people across the United States in January 2016, nearly 15,000 fewer than there were in the first month of 2015, a 3% decline.

The U.S. Department of Housing and Urban Development delivers to Congress annual estimates of homelessness across the nation. Based on the latest data from HUD, 24/7 Wall St. reviewed the share of each state’s homeless population that is unsheltered. These are individuals who are homeless, but are not staying in emergency shelters, transitional housing programs, or safe havens.

California has the largest share of homeless residents who are unsheltered, at 66.4%. Washington state rounds out the list of 10 states with the largest shares of unsheltered homeless people — there, 40.6% of homeless residents are unsheltered. For reference, an estimated 32.1% of the 549,928 homeless individuals nationwide are unsheltered.

Click here to see the states with the highest shares of unsheltered homeless people.

Generally speaking, people become homeless because of strained financial resources and the challenge of finding affordable housing. Such factors as cost of living and median home value are therefore closely related to the incidence of homelessness. The cost of living in eight of the 10 states with the highest shares of homeless people — sheltered or unsheltered — is higher than the national average, and the median home value exceeds the national median of $194,500 in all 10 states without exception.

The homeless population is perhaps the nation’s most vulnerable group of people. Community members, non-profit organizations, state governments, and federal programs all work to lower the incidence of homelessness and help the homeless population, starting with providing temporary shelter. And while states with the largest unsheltered homeless populations tend to have high rates of overall homelessness, this is not always the case. In five of the 10 states with the largest unsheltered homeless populations, the ratio of homeless people to total population is lower than the national average of 1 in every 588 people.

The characteristics of the nation’s unsheltered homeless population sheds light on the conditions that can lead to more unsheltered homelessness. Compared with their sheltered counterparts, unsheltered homeless people are more frequently located in areas with warmer temperatures. People living in unsheltered situations are more likely to be veterans, have a history of incarceration, have lower levels of education, and significant substance use and mental health histories, according to the 2016 study in the U.S. National Library of Medicine, Homelessness, Unsheltered Status, and Risk Factors for Mortality: Findings from the 100,000 Homes Campaign.

Dr. Dennis Culhane is the Dana and Andrew Stone chair in social policy at the University of Pennsylvania and one of the principal researchers on HUD’s annual homelessness report. In an interview with 24/7 Wall St., Culhane said that in the areas with large unsheltered homelessness it is “not that there’s more homelessness, it’s that there is less shelter effort.” Even more than climate factors, the culture of helping and the general willingness among a population to assist people in need may account for the presence of temporary housing programs in an area, according to research Culhane continues to conduct across the United States.

Culhane’s hypothesis connects the current numbers of shelters in a state with the area’s broader traditions of social welfare provisions. Over time, philanthropic institutions such as churches, emergency housing programs, and even unions laid the foundation for the current high rates of sheltered homeless people nationwide. Culhane argued that this tradition is linked to the economic composition of states going back at least a century. In some, particularly southern and western states, there were fewer and less generous social welfare programs due to their economies: agriculture in the South and migratory workers in the West. “[T]hey wanted people to be compelled to work at the low wages that prevailed.” As a result, the infrastructure for supporting the homeless was not in place and sheltering efforts remain largely insufficient in these areas today.

All of the 10 states with the highest shares of unsheltered homeless people are in either the southern or western United States.

Numerous studies have demonstrated a link between rates of sheltered homeless people and safety net factors such as Medicaid spending. Medicaid can certainly have a direct impact on homeless individuals in need of health care, and it is also an approximation of the strength of social welfare programs in a state. Annual Medicaid spending per enrollee exceeds the national average expenditure of $5,790 in just three of the 10 states with the highest shares of unsheltered homeless people.

Click here to read our methodology.

Here are the states with the most unsheltered homeless people.