In the case of Occidental Petroleum's deal for Anadarko Petroleum, the calculation is complicated by rumours and a bid battle with Chevron that preceded the deal, but however you cut it the shares now languish close to a 10-year low.
You can attribute some decline to "merger-arbs" — hedge funds which typically buy the target and sell the acquirer — but such significant falls cannot be so easily explained away.
In other cases, shareholders are choosing to stand and fight. Carl Icahn this week intensified his battle with the board of Occidental, calling for four directors to step aside.
The veteran activist, who owns 5 per cent of Occidental's shares, worth US$1.6b, has already sued the company over its "hugely overpriced" acquisition.
His particular bugbear, which is compelling, is that Occidental strained every sinew to avoid a shareholder vote. If it is so convinced that it is worth outbidding Chevron to snap up Anadarko's shale assets then it should let shareholders have their say.
But however valid his argument, Icahn's readiness to opine so colourfully and so frequently lessens the impact of any individual intervention.
More powerful is the opposition of long-only institutions. Scepticism about deals among big asset managers has always been out there. It is just usually communicated privately.
"You have let us down" is the title of an email from a portfolio manager at Columbia Seligman to HP's chief financial officer, just after the computer company announced its US$11b acquisition of Autonomy, the UK software maker, in 2011. That is just one of a series of blunt rebukes from institutional investors that only became public thanks to one of the various court cases that have shed light on the calamitous deal.
Increasingly, though, investors are deliberately raising their head above the parapet. Wellington Investment Management, the top shareholder in Bristol-Myers Squibb, put out a press release in February to say that it did not support the pharmaceutical company's proposed Celgene acquisition.
In Occidental's case it is not just Icahn who is exercised. T Rowe Price, a usually placid asset manager, has labelled the price "extraordinarily expensive" and said "we don't feel we have any choice" to vote against the board's re-election given its refusal to grant a vote on the deal.
The level of acrimony requires a response. Managements might spend more time mollifying shareholders or — in the US and other jurisdictions where this allowed — try to ignore them. But continued sell-offs and fights will have a deterrent effect: less elephant hunting.
Written by: Tom Braithwaite
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