The confidence of venture capital investors in Silicon Valley is continuing to slide, according to a new survey.

Reasons for the drop include continuing fears over the huge valuations of some private companies known as unicorns and the larger global macro-economic picture. At the top of the list of worries are the Greek debt crisis and its impact on the euro zone, as well as the stock market drop in China. Another big concern is the high cost of doing business in Silicon Valley.

That’s according to the Silicon Valley Venture Capital Confidence Index, whose data on the second quarter of 2015 was released Tuesday. The Index scores investor confidence on a scale of 1 to 5, with 5 being the highest. In the second quarter, confidence decreased to 3.73 from 3.81, the second consecutive quarter of decreases. The average for the survey for the past 11 years has been 3.73, according to the survey authors.

“Expectations are beginning to outpace reality,” venture capitalist Bob Ackerman, founder and managing director of Allegis Capital in Palo Alto, says in the release. He said that unicorn valuations are having "knock-on effects for the venture ecosystem" in valuations, compensation, and generally the cost of doing business for venture companies. 

In recent years, up to 100 private companies have achieved $1 billion valuations, with some far outpacing that. For example, car-share company Uber has a valuation of $50 billion. House and apartment-share company Airbnb is valued at $24 billion. And big-data-crunching security software company Palantir has a valuation of $20 billion.

“Both seed-stage and late-stage venture are experiencing frothiness as the tourists have arrived, and they are paying prices that the locals won’t,” Venky Ganesan, a partner at Menlo Ventures, of Menlo Park, California told the survey authors. He expects a near-term correction when the Federal Reserve raises interest rates, but his long-term outlook is strong.

Overall, however, venture capitalists expressed optimism for the rest of the year, particularly around exits. Primarily, initial public offerings and mergers and acquisitions should remain strong. Global merger-and-acquisition activity for 2015 has reached $2 trillion, the report says.