NEW YORK (TheStreet) -- Exchange-traded funds are on track for another strong year, with year-to-date inflows totaling $72.5 billion. This year's inflows, which have pushed industry assets to more than $2.12 trillion, are more than simply about investors fearful of single stock risk, said David Mazza, head of research at SPDR ETFs and SSgA Funds at State Street Global Advisors. 

"It's also just the growth of the ETF industry," said Mazza. "It continues because of the diversification benefits," including liquidity and transparency, offered by buying these bundles of stocks that track particular indexes or sectors. 

According to Mazza, attractive valuations in international markets and stimulative economic policies in Europe and Japan had ETF investors looking beyond U.S. borders during the first half of 2015. Currency Hedged ETFs and Broad International ETFs saw inflows of $34.5 billion and $13.4 billion respectively, according to SSgA.

"People have been following the money but what they have really been following is the quantitative easing. That's what we've seen in Europe, in Japan and, in particular, when [European Central Bank chief] Mario Draghi comes out and says, 'we will do whatever it takes.' That's where the money has flowed," said Mazza. "If you look at the valuation opportunities there, Europe actually continues to be favorable even though there is a lot of headline risk from Greece or wherever it might be from."

Meanwhile, U.S. equity ETFs experienced outflows of $26.2 billion so far this year, even though the S&P is up nearly 3% and the U.S. economy is viewed as the strongest among developed markets.

"Investors have been ignoring the United States a bit because of all the valuation opportunities and all the extraordinary monetary policies that have been popping up overseas, but let's not forget there has been some strength that's actually coming into our economy," said Mazza. "That continues to propel forward the ability for American companies to have favorable earnings growth."

Perhaps surprisingly, Energy ETFs have seen the biggest inflows in 2015 with $6.5 billion. That comes after a rough ride at the end of 2014 when oil prices fell off the table.

"Energy was a rough ride in 2014, but in 2015 investors have actually looked for a bottom, the opportunity to step in. There's not a lot of value that folks can find across the markets because they have been on a strong rally but one clear spot this year has been energy," said Mazza. "Looking ahead, I think investors will be a bit more cautious potentially because of where oil might run, but near term they continue to like the space."

Finally, bond funds saw inflows of $26.3 billion in 2015, despite the 10-year Treasury yield ticking up from 1.7% in January to more than 2.4% in the wake of the impressive May Jobs report.

"Some of that has to do with the fact that fixed-income ETFs offer liquidity in a market that might not be as liquid as others," said Mazza. "But they are also not necessarily going into Treasuries. They are looking at high yield and investment grade corporates, even senior loans and convertible bonds now to build that diversification that they need particularly with rising rates potentially right around the corner."

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