The definite etiology of MBC is unknown. Factors such as alteration in hormonal milieu, family history and genetic alterations are known to influence its occurrence. Various studies have shown that conditions that alter the estrogen-testosterone ratio in males predispose to breast cancer [
14,
15]. Among these conditions the strongest association is with Klinefelter's syndrome. Males with this condition have a fifty times increased risk and accounts for 3% of all MBC [
16]. Conditions, which are associated with increased estrogen levels, like cirrhosis [
17,
18] and exogenous administration of estrogen (either in transsexuals or as therapy for prostate cancer) have been implicated as causative factors [
19‐
22]. Also, androgen deficiency due to testicular disease like mumps, undescended testes, or testicular injury, has been linked to the occurrence of breast cancer in men [
23,
24]. Occupational exposure to heat and electromagnetic radiation, causing testicular damage and further leading to the development of MBC is also postulated [
25,
26]. An inherited predisposition for breast cancer is noticed in males-analogous to that in females [
27‐
31]. A positive family history of a first-degree female relative having breast cancer is seen in up to 15–20% patients [
32]. This increased risk is conferred by mutations in the breast cancer susceptibility genes (BRCA1 and BRCA2). Mutations in both the BRCA1 and BRCA2 genes are linked to female breast cancer. Genetic studies in males however, have shown that germline mutations in BRCA2 alone account for the majority of hereditary breast cancer [
33‐
36]. No link between BRCA1 and familial breast cancer has been noticed in one study [
37], whereas other studies have suggested a possible link [
38,
39]. The Cambridge study showed that 8% of patients had BRCA2 mutations and all the carriers had a family history of breast, ovarian, prostate or pancreatic cancer [
40]. The highest prevalence of BRCA2 mutation in MBC has been noted in Iceland where 40% have the mutation [
41]. Several case reports have linked MBC with other genetic disorders like Cowden syndrome [
42] and Hereditary Non-Polyposis Colonic Cancer (HNPCC) [
43]. It has been recently reported that male breast cancer may also predispose to increased risk of developing a second cancer of the stomach, skin and breast [
44].
A strong racial predilection is noted in MBC, with studies establishing a high-risk for Jews. Among them, the Sephardic Jews present at a younger age with advanced stage disease whereas the Ashkenazi Jews have an increased lifetime risk of suffering from the disease [
45,
46]. Gynecomastia, present in 6–38% of MBC patients has also been implicated as a risk factor [
47,
48] and some studies have shown positive correlation between the two [
49]. An interesting study in the US comparing incidence, pathology and outcomes in male and female breast cancer in a defined population showed more black males than white males to be affected. Also black men with breast cancer had more involved axillary lymph nodes and higher stage than whites at presentation [
50]. This is in stark contrast to the high incidence of male breast cancer preponderance in whites as shown in another recently reported study in the US which showed higher incidence in white males, although black males were more not likely to see an oncologist for consideration of chemotherapy and had higher mortality associated with the disease (hazard ratio = 3.29; 95% CI, 1.10 to 9.86) [
51]. Reports have shown that an association of MBC and gynecomastia could also represent a chance occurrence as 35–40% of healthy men have clinical or histological gynecomastia [
52].
Alcohol has been variably linked as a causative factor in the genesis of MBC. A large Swedish study has not shown any such correlation [
53], although it has been implicated as a causal agent in other studies [
54]. A case control study conducted in Europe has shown that for alcohol intakes of less than 60 grams per day, the relative risk of MBC is comparable to that in females, and it continues to increase at high consumption levels [
55]. Other risk factors mentioned in various studies are low socioeconomic status, obesity, pacemakers, tuberculosis and hyperthyroidism [
56,
57]. A meta analysis of 7 case-control studies revealed that the risk of MBC to be significantly increased in males with the following characteristics; never married, benign breast disease, gynecomastia, Jewish or history of breast cancer in a first-degree relative [
58‐
61].