Compared to their large-cap counterparts, mid-cap stocks historically show higher rates of returns. Yet, this better performance comes at an expense of higher volatility. Mid-cap stocks typically have higher growth rates and better investment prospects compared to large-cap stocks that may experience market saturation. At the same time, firms of mid-cap stocks tend to have better competitive advantages and more financial resources compared to their small-cap counterparts. Investors can gain diversified exposure to mid-cap stocks by buying passively managed mutual funds that follow an indexing approach.

Vanguard Mid-Cap Index Fund Investor Shares

Assets Under Management (AUM): $67 billion

2010-2015 Average Annual Net Asset Value (NAV) Return: 13.46%

Net Expense Ratio: 0.23%

The Vanguard Mid-Cap Index Fund Investor Shares tracks the performance of the CRSP US Mid Cap Index, a broadly diversified index of stocks of mid-size U.S. companies. The fund invests in all components of the underlying index instead of following a sampling approach. It has a relatively low turnover ratio of 11%, making it highly tax-efficient. Financial stocks have the largest weight in the fund's portfolio with about 20%, while industrial stocks account for 17% of the fund's assets. No single holdings account for more than 1% of the fund's assets.

Due to its exceptionally low-cost structure, low tracking error and experienced management, the fund has earned a gold analyst rating and a four-star overall rating from Morningstar. It comes with no load fees and requires investors to contribute at least $3,000 as an initial investment.

Dreyfus Mid Cap Index Fund

AUM: $3.1 billion

2010-2015 Average Annual NAV Return: 12.51%

Net Expense Ratio: 0.50%

The Dreyfus Mid Cap Index Fund tracks the performance of the widely followed Standard & Poor's MidCap 400 Index, which is composed of 400 common stocks of U.S. medium-size companies. The fund uses a full replication strategy by holding all 400 stocks included in the benchmark index. No single sector in the fund's portfolio has an allocation in excess of 11%. The real estate sector has 11% allocation, while capital goods stocks account for 10% of the fund's assets.

The fund's primary manager, Thomas J. Durante, has been with the Dreyfus Mid Cap Index Fund since 2000 and was able to beat the average risk-adjusted returns of his peers in the mid-cap blend group. Morningstar has awarded the fund with a four-star overall rating. The fund does not have load fees and comes with a minimum contribution amount of $2,500.

Fidelity Spartan Mid Cap Index Fund Investor Class

AUM: $1.4 billion

2012-2015 Average Annual NAV Return: 15.85%

Net Expense Ratio: 0.22%

The Fidelity Spartan Mid Cap Index Fund Investor Class seeks to track the performance of the Russell Midcap Index, which is composed of the smallest 800 stocks included in the Russell 1000 Index. Most companies held by the fund are of medium market capitalization. Financial stocks occupy the largest place in the fund's portfolio with about 22% allocation. Consumer discretionary stocks and information technology equities have 17% and 15% allocations, respectively. The fund's holdings are very diversified with no single holding having more than 1% of the fund's assets. The fund's top 10 holdings have 4.2% allocation.

The Fidelity Spartan Mid Cap Index Fund Investor Class has one of the lowest costs among its peers. It has earned a four-star overall rating from Morningstar. The fund does not charge load fees and requires a $2,500 minimum investment.

Columbia Mid Cap Index Fund Class A

AUM: $3.7 billion

2010-2015 Average Annual NAV Return: 12.54%

Net Expense Ratio: 0.45%

The Columbia Mid Cap Index Fund Class A tracks the performance of the Standard & Poor's MidCap 400 Index. Like its Dreyfus peer, this fund pursues full replication and holds all 400 stocks in its portfolio. It has a slight advantage compared to its Dreyfus rival in terms of a slightly lower expense ratio, which could add up to significant numbers long term. The Columbia Mid Cap Index Fund Class A has earned a four-star overall rating from Morningstar. It comes with no load fees and requires a $2,000 minimum investment.