In Fortune magazine, Erin Griffith put together a chart exposing "Silicon Valley's single degree of separation," where A-listers are "just an adviser, an investor, a co-founder, an acquirer, or a director away from another."

That small town clubbiness in a trillion dollar industry has invited Silicon Valley's most shrugged off criticism: conflict of interest.

About one-fifth of all corporate directors in the Fortune 500 sit on more than one board, according to 2010 research by G. William Domhoff. But it's standard practice for venture capitalists to join the board of a company when they lead an investment round, and typical for them to do a handful of deals each year. Do the math, and the possibility of double-digit board appointments is not surprising.

The effects of this practice — call it board-whoring — isn't usually a problem with small, early-stage startups trying to gain their footing. But it can become an issue when those tiny startups turn into large public entities with billions of dollars of shareholder money at stake. In the technology industry, a convergence of strategies can occur almost overnight.

It can also "become an issue" when activist investors like Carl Icahn decide to make it an issue. Icahn recently attacked Marc Andreessen for double-dipping in eBay's Skype deal. In response, Andreessen went to town on Icahn in a series of blog posts, one of which referred to himself in the third person.

Andreessen, who is on the board of Facebook and eight other companies, also engaged in some shady social media this week when one of his investments got bought by Facebook. When the $2 billion deal was announced, Andreessen said he played no part, "in keeping with standard guidelines for board members, who are supposed to be working objectively for shareholders."

However, as Marketwatch points out, Andreessen spent the day of the Oculus acquisition tweeting about the new Oculus Rift device.

That discussion was helpful because Oculus was back in the news Tuesday afternoon as Facebook FB +1.04% agreed to buy the firm for $2 billion, mostly in Facebook stock.

Andreessen must not have wanted those tweets to be used "For the record," because he deleted them soon after.

Fortune talked to one expert who said venture capitalists may not even know they're acting unethically:

"Human beings tend to be unable to estimate how biased they are," says Jean-François Manzoni, a professor of management practice at Insead, the French graduate business school. "Research on ethics and decision-making shows fairly clearly that even minute incentives that one has in the outcome of a decision can have significant impact on people's decision-making."

A certain subspecies of human beings seem particularly bad at estimating bias.

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