<p>Sovereign investors are sinking more money into tech start-ups and opening offices in Silicon Valley in the hope of bagging a &quot;unicorn&quot; - the rare private firm that grows in value to over $1 billion.</p> <p>Sovereign wealth funds (SWFs), which run over $6.59 trillion in assets, hope that investing at an early stage will yield outsize returns if the firms enjoy dizzying growth.</p> <p>But such start-ups can also offer a useful hedge in case SWF holdings in mature sectors come under threat from digital disrupters, just as global hotel chains have been undercut by online room rentals business Airbnb - itself a unicorn.</p> <p>SWFs made 12 investments last year in U.S. start-ups, worth $12.4 billion, up from four investments in 2012 worth $202 million, according to data compiled by research firm PitchBook.</p> <p>Globally, there were 42 deals involving SWFs and start-ups last year valued at some $16.2 billion, according to the Sovereign Wealth Lab research center at Madrid&#39;s IE Business School.</p>