Conventional investor wisdom has it that a majority of bank mergers don't realize their promised synergies— leaving both the banks and their investors disappointed. Yes, there is plenty of anecdotal evidence of "in-market" mergers between two banks from the same country that did not live up to the hype.

What's interesting is that when it comes to international mergers, the story is much different. In fact, recent research by our firm shows that the most successful mergers occur when Western banks take an ownership stake in emerging-market banks.

There is logic to this: Emerging markets tend to have low penetration rates for banking services, which gives Western banks an opportunity to grow at a higher rate than is possible in their home markets. For example, loan penetration is below 50 percent in the Czech Republic, Poland, Argentina, Brazil, Egypt and Indonesia, vs. the 100 percent-plus rates for developed markets such as the U.S. and U.K.

What's more, emerging markets present global banks with the opportunity to acquire operations in developing locales at lower prices and then use their management expertise to help the emerging market banks improve their operations. Emerging markets also offer a lower-cost platform for global banks to do business, especially since wages typically comprise at least 40 percent of a global bank's cost structure.

According to our research, 12 of the 15 deals where the acquired banks enjoyed the biggest post-merger revenue gains were from emerging markets and the seven deals where the target enjoyed the highest stock returns following the merger were from emerging markets as well. All of this suggests that U.S. banks would be well-served to seek out partners in emerging markets such as Brazil or Indonesia.

To assess the long-term performance of cross-border bank mergers, we analyzed 89 deals announced between 2000 and 2009. To appraise the success of each deal, we measured the performance of the target banks by five financial metrics: the stock performance during the first year after the deal; annual revenue growth; after-tax return on equity; the efficiency ratio (expenses as a percentage of revenues; and the change in market value.

Our finding: Most of the cross-border deals — particularly those where the acquirer assumed majority control — also paid off for the target bank. By our count, 42 percent of the banks acquired in cross-border mergers recorded higher profits and nearly 70 percent had a higher stock price one year after the deal.

The secret? An easy explanation is the savings these banks generated from combining redundant operations. Indeed, slightly more than half of the acquired banks boosted their efficiency ratio in the year after the deal—and by an average of five percentage points.

But this rise in profitability among the acquired banks wasn't just the result of cost-cutting: Emerging-market banks that sold a majority stake saw their revenues soar afterwards, thanks to the broader product offerings and marketing support provided by their new partners. The result: revenues for emerging-market institutions grew at a compound annual rate of 18.7 percent following their mergers, vs. the 14.6 percent revenue growth for acquired banks from developed economies.

The M&A experience of some Western banks shows that cross-border deals provide long-term value for themselves and the banks they acquire. Some of the more successful acquirers were able to apply their experience from successful mergers to other parts of the region. For example, several Spanish banks purchased banks in Puerto Rico in the 1970s, which prepared them for later acquisitions in Latin America.

Having developed extensive cross-border acquisition integration capabilities, both BBVA and Santander have since been able to sustain their growth by investing in markets outside of Latin America. For example, BBVA has emerged as a leading bank in the Sun Belt region of the U.S. following its acquisitions of Birmingham-based Compass and a number of Texas banks.

At the same time, both banks also extended their reach across the rest of Europe. (For its part, Santander acquired three U.K. institutions: Abbey National, Alliance & Leicester and Bradford & Bingley.) As a result, Santander and BBVA both earn a surprisingly large share of their profits from outside their home market—once again showing how reality can be quite different than the myths would have it.

Bruce Kiene is a senior executive in Accenture's management consulting practice who advises financial institutions. David Helin is a research executive in Accenture’s financial services group. They are authors of a recently released study, "Cross-Border Banking: An Accenture Study of Cross-Border Mergers & Acquisitions in Banking."