Being obese increases a man's risk of breast cancer by nearly a third, research has shown.
Other risk factors include enlarged male breasts, and having an extra X chromosome besides the one men normally inherit from their mothers.
Around 400 men are diagnosed with breast cancer each year in the UK, but the causes of male breast cancer are poorly understood.
The Male Breast Cancer Pooling Project gathered together data on the disease from studies conducted around the world.
It found that obesity, and several other physical and hormone-related traits, were associated with increased breast cancer risk in men.
The results published in the Journal of the National Cancer Institute (JNCI) showed that breast cancer risk in men increases with weight.
Clinical obesity, meaning a Body Mass Index of 30 or above, increased the chances of a man developing breast cancer by around 30%.
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An association was also seen with gynecomastia, enlarged breast tissue in men, which appeared to be separate from the effect of obesity.
Klinefelter syndrome, the presence of an extra X chromosome in men, was another confirmed risk factor.
Professor Anthony Swerdlow, from the Institute of Cancer Research in London - who led the UK arm of the study, said: "This research brings together data from studies of male breast cancer from around the world to clarify risk factors that have been uncertain.
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"The results suggest that men who are overweight may be at increased risk of male breast cancer.
"We know that body size can be related to hormone levels. Also, hormonal factors may be the reason why patients with Klinefelter syndrome, who have comparatively low levels of testosterone and high levels of oestrogen, have raised breast cancer risks compared with other men.
"Our results suggest the need to investigate further the role of sex hormones in causation of breast cancer in men."
Dr Matthew Lam, from the charity Breakthrough Breast Cancer, said: "We know that hormonal factors play a large part in increasing risk of breast cancer in women but how these factors affect risk in men is not well understood.
"This study provides new insight into the contributing role of sex hormone levels and physical conditions that control them in male breast cancer. "
Breakthrough-funded scientists have previously identified a gene called RAD51B that can cause breast cancer in men.