The long-running reliance by Australia's banks on cross-selling to boost revenue may well be a flawed strategy, according to the latest 'share of wallet' data published by Roy Morgan.  In this context of the Roy Morgan data, "wallet" refers to the total value of products - whether savings, loans or wealth management products - held by an individual customer.The term "share of wallet" is used to assess how successfully a company or group is referring existing customers to other product lines within the group - a practice the banks have been heavily criticised for by the Hayne royal commission.The Big Four banks have managed to maintain a larger share of their customers' total number of financial products than their smaller rivals. CBA Group on average provides three out of 8.2 products held by each customer; ANZ provides 2.8 out of 9.6 products; NAB provides 2.7 out of 9.8 and Westpac provides three out of 9.6.This compares favourably with smaller banks such as ING, which captures an average 2.2 out of 11.9 products held by its customers; Bendigo Bank which provides 2.3 out of 8.8; and Westpac's multi-branded subsidiary St George, which provides an average 2.6 out of 10.1 products held each customer.In particular, banks have been accused of aggressive cross-selling of products such as superannuation or life insurance to mortgage holders or financial advice clients.Nevertheless, the Roy Morgan Banking and Finance Industry Currency Report found most major banks' 'share of customer wallet' has declined over the past four years, with the National Australia Bank Group slipping 3.7 percentage points to 29.5 per cent (ie, of the total value of all products held by its customers), CBA losing 2.6 percentage points to sit at 31 per cent share of wallet, ANZ Group sliding 1.1 percentage points to 27.7 per cent and Westpac down 0.9 percentage points to 28.3 per cent.While there is no doubt the royal commission has exposed serious problems in the selling practices of banks, the decline in share of wallet highlights the increased levels of competition across a number of product categories. "What our latest data reveals is that consumers are looking around for new products and new providers of those products," observed Norman Morris, Roy Morgan industry communication director."Competition is clearly increasing in the financial services sector, not least because of the rise of highly innovative fintech offerings."This view is reflected in the submission from the Reserve Bank of Australia to the Productivity Commission Inquiry into Competition in the Financial System.