The specific cause of rheumatoid arthritis (RA) is unknown. Although the immune system attacks the tissue in the joints, no one knows what triggers such an autoimmune attack, or why it first attacks only a few joints. Some researchers think that some outside factor or a combination of factors trigger the initial inflammation.

Risk factors are elements that may increase a person’s likelihood of developing a disease or condition. For RA, risk factors may be divided into two groups. Susceptibility factors may indicate a person is susceptible to the disease. Initiating factors are those that may occur with the susceptibility factor and trigger RA. The presence of a risk factor does not mean a person will develop the condition, and the absence of a risk factor does not mean the condition will not occur.

Susceptibility risk factors for RA include:

• Sex. Like many other autoimmune diseases, RA is much more common in women. Women make up about 70 percent of the RA cases in the United States, according to the Arthritis Foundation (AF).
  
  
• Age. RA can occur at any age but usually begins between the ages of 30 and 50.
  
  
• Family history and genetics. Having immediate family members with RA (parents, siblings or children) increases the likelihood of developing the disease. One of the genes associated with RA is HLA-DR4. This gene is present in some percentage of the entire population, but they may not develop RA. However, about two-thirds of Caucasian RA patients have this gene, according to the AF. Presence of another autoimmune disease. People with an autoimmune disorder, such as Sjogren's syndrome, type 1 diabetes or lupus, are at greater risk for developing others.

Birth weight might also affect susceptibility. Women in the U.S. Nurses' Health Study were twice as likely to develop RA if they weighed more than 10 pounds at birth.

Known or suspected initiating risk factors for RA include:

• Smoking. People who smoke have a higher risk of developing RA. Smoking may also be a factor that initiates inflammation.
  
  
• Infection. A previous infection with bacteria or a virus may help initiate RA. The link is suspected, but there is no evidence yet to support it.
  
  
• Lifestyle factors. Stress and diet may also be factors that can initiate RA inflammation, but no studies have proven this as of yet.