Yesterday, Ed Miliband attacked 'short-termist culture' and people who chase a fast buck. This is a mistake. There's much to be said for short-termism.
The evidence that investment is being held back by short-termism comes from a recent paper (pdf) by Andrew Haldane and Richard Davies. They say:

Cash-flows 5 years ahead are discounted at rates more appropriate 8 or more years hence; 10 year ahead cash-flows are valued as if 16 or more years ahead; and cash-flows more than 30 years ahead are scarcely valued at all.

But I'm not sure this is irrational or even wrong. This is because Knightian uncertainty increases very sharply as we look further ahead.
Put yourself in the shoes of a CEO looking one or two years ahead. He's got a fair idea of how Porter's five forces (or another other framework for analyzing business prospects) will look. New products or powerful new rivals don't usually emerge that quickly.
But if he looks 10-20 years ahead, he'll not have a clue. We can't predict how technology and market power will shift over such horizons.
This means that whilst we might have some idea about investment paybacks over one or two years, we can't have such firm ideas over 10-20 years. In this context, short-termism is reasonable.
If we'd had long-term investment in manufacturing in the 80s, we'd have built lots of factories making fax machines and instant cameras.
And in the early 70s, British Rail's long-term thinking caused it to patent a space ship. Its customers would probably have preferred a little more short-term investment in the 7.52 from Orpington.
I suspect that a lot of what looks like long-term investment is in fact short-termism that got lucky. When Bill Gates started Microsoft in the early 80s, it wasn't because he was preparing for a boom in computer demand 15 years hence. It's because he saw a little niche and lucked out as the niche grew.
What's more, short-termism can protect us from two cognitive biases.
One is overconfidence. Given that the long-term future is unknowable, investment in long-term projects is often founded upon overconfidence about one's predictions; this might be true for high speed rail. Short-termism offsets this.
The other bias is the planning fallacy. Complex projects take longer and cost more than expected. Had we had a little more short-termism, we'd not have wasted hundreds of millions on rejigging NHS IT systems or on building a tram system in Edinburgh.
The record, then, surely shows that governments shouldn't encourage long-termism. Its own long-termism is bad enough.
This is all the more true if we ask: what can governments do to ensure that companies can earn long-term returns? The most effective thing they can do is to reduce the uncertainty firms face about future competitive threats. But this requires the government to protect vested interests - which is exactly what Mr Miliband pledged not to do.