Countries such as China, Mexico and Brazil often steal the headlines when it comes to emerging markets. The rise of the middle class in these nations presents mouth-watering investment opportunities that many investors don't want to pass up.

Investors should also consider investing in "frontier markets" – markets that are more established than less-developed countries (LDCs) but smaller than traditional emerging markets. In January 2018, frontier-market stocks had a combined market capitalization of over $700 billion, their highest value in a decade, and have begun to outperform emerging-market stocks for the first time in three years as reported by Bloomberg. (For more, see also: The Difference Between Emerging and Frontier Markets.)

Catalysts for the increased appetite in frontier markets relate to a number of macroeconomic reforms in these smaller developing economies, such as the substantial overhaul to Argentina's taxation system, a push for privatization in Vietnam and currency liberalization in countries like Morocco and Nigeria. The head of equities at Blackfriars Asset Management Ltd. in London told Bloomberg, "Strengthening oil prices may be one broad theme, but otherwise, there are specific elements like currency reforms that are driving specific markets."

Investors who want to add frontier-market stocks to their portfolio can do so cheaply by investing in one of these three exchange-traded funds (ETFs). (See also: 5 Emerging Market Equity ETFs to Watch in 2018.)

iShares MSCI Frontier 100 ETF (NYSEARCA: FM)

Launched in 2012, the iShares MSCI Frontier 100 ETF aims to provide similar returns to the MSCI Frontier Markets 100 Index. The fund does this by investing the majority of its assets in securities that are constitutes of the underlying index. This index tracks untapped and high-growth markets as classified by MSCI Inc. (NYSE: MSCI). Key allocations in the fund's portfolio include National Bank of Kuwait SAKP (KW: NBKK) at 5.93%, Vietnam Dairy Products JSC (HCM: VNM) at 4.44% and Kuwait Finance House KSCP (KW: KFH) at 4.36%. The ETF's basket holds 118 stocks in total.

The iShares MSCI Frontier 100 ETF has assets under management (AUM) of $565.9 million. Its 0.8% expense ratio is higher than the category average of 0.58% but offset by an attractive 4.19% dividend yield. FM has returned 4.09% over the past five years and 1.93% over the past three years. Year to date (YTD), the fund has returned -10.65% as of August 2018. (For more, see: When Is an Expense Ratio Considered High and When Is It Considered Low?)

Invesco Frontier Markets ETF (NYSEARCA: FRN)

The Invesco Frontier Markets ETF, formed back in 2008, attempts to mirror the performance of the BNY Mellon New Frontier Index. The fund achieves this by investing the lion's share of its assets in securities that make up the tracked index. It holds both liquid American depository receipts (ADRs) and global depository receipts (GDRs) of securities from frontier countries. The ETF's portfolio contains 68 stocks. FRN's top three holdings – Copa Holdings SA Class A (NYSE: CPA), KAZ Minerals PLC (OTC: KZMYF) and Safaricom PLC (NR: SCOM) – carry a combined weighting of 20.55%.

The Invesco Frontier Markets ETF has $66.19 million in net assets and charges investors a 0.7% annual management fee. As of August 2018, the fund has 10-, five- and three-year annualized returns of -2.93%, 0.3% and 4.61%, respectively. It has returned -7% YTD and pays a 3.86% annual dividend. (See also: What Are Frontier Markets?)

Global X MSCI Argentina ETF (NYSEARCA: ARGT)

Created in 2011, the Global X MSCI Argentina ETF seeks to replicate the returns of the MSCI All Argentina 25/50 Index. To do this, the ETF invests at least 80% of its assets in securities that make up the benchmarked index. The fund holds companies that are headquartered in Argentina or conduct the majority of their operations in the country. Top holdings in the ETF's portfolio include MercadoLibre Inc. (NASDAQ: MELI), Tenaris SA (NYSE: TS), Grupo Financiero Galicia SA ADR (NASDAQ: GGAL) and Globant SA (NYSE: GLOB).

The Global X MSCI Argentina ETF has AUM of $146.44 million and charges a 0.65% management fee. Although the fund has a disappointing YTD return of -23.62%, it has performed better over the longer term. Over the past five years, the ETF has returned 11.44%, while over the past three years, investors have enjoyed an 11.04% return. ARGT pays a 0.65% dividend yield, effectively offsetting the annual management fee. (See also: 3 Ways You Can Invest in Argentina From the U.S.)

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