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For decades people have been interested in the question of whether stress plays some role in cancer's onset and progression. To get to the point, the answer right now is that we don't really know, but there are some interesting nuances around this question. Typically this type of research looks at whether stress is related to cancer incidence (who gets cancer) or progression (once you have cancer, who survives).

The best way to determine if stress is related to the development of cancer is to gather a big group of people, assuming that in time some of them would go on to contract various illnesses, and ask them a lot of questions. You would want to ask about their stress levels, mood, family
history of illness, and other risk factors for cancer, and then just wait to see what happened. Inevitably some will go on to develop cancers.  Then you would have to be very careful to take into account all the other known risk factors for cancer, especially since some of them are also associated with higher stress levels or certain personality traits, including things like diet, smoking, drinking alcohol, physical activity, exposure to environmental and occupational carcinogens, family history, and genetics.
     
Several big studies of this nature have been conducted and reviewed. In his paper "Psychological Factors and Cancer Development: Evidence After 30 Years of Research," Bert Garssen (2004) summarized 70 such studies. Garssen broke down the vague category of "stress" into discrete dimensions that were often measured in these studies. The areas of interest were stressful life events (including a wide range of events like divorce, illness, and job loss right down to holidays and children leaving home); loss events (losing a child or spouse); social support; quality of partner relationship; personality; coping styles; and measures of distress, depression, and psychiatric diagnoses. Each of these factors had been theorized to potentially impact cancer development.

So what did Garssen find? There was no clear evidence that stressful life events, bereavement and loss, social relations or distress, and depression symptoms were related to getting cancer, even though in each area several studies found relationships to be a factor. For example, of 14 studies that looked at whether depression levels predicted later cancer onset, seven found that depression did predict cancer initiation, but seven found that depression did not! This shows how important it is to review a whole body of literature, rather than just rely on one or two studies.

Whichever view you hold, there's research available to support your contention. The truth is that there's likely no one right answer; perhaps in some cases, for certain individuals or types of cancers, a factor may be causal, but not in others.

Adapted from Carlson, L.E. & Speca, M. Mindfulness-Based Cancer Recovery: A Step-by-Step MBSR Approach to Help You Cope with Treatment and Reclaim Your Life. 2011. New Harbinger, CA.