Think of It as a Vote of Confidence in Yahoo’s Turnaround Plan, Jerry. You’ll Feel Better.

Add Capital Research & Management Co. to the list of Yahoo (YHOO) shareholders apparently convinced the company will be acquired by Microsoft (MSFT). In a regulatory filing published yesterday, the company revealed that it had nearly doubled its stake in Yahoo.

On Dec. 31, 2007, Capital held 5.2% of Yahoo’s outstanding shares. As of March 31, it holds 10.1%. Now, while it’s possible that the company could have increased its Yahoo stake before Microsoft’s acquisition offer Feb. 1, it’s more likely that it increased it afterward, betting that the software giant would be forced to raise its bid to close the deal.

“That was smart,” analyst David Hilal said of the move, which makes Capital the largest Yahoo shareholder. “I’m still of the belief that Microsoft is going to buy Yahoo and they’re going to raise their bid. And I’m assuming they’re making the same bet over there [at Capital].”