When we read Ian Hurst's claim that half of senior IRA members and a quarter of more junior ones were agents, it causes a swift intake of breath. However, it shouldn't really be that surprising.
In a long conflict, the interactions between the two sides are bound to be pretty complex and many people will pass information to the other side for one reason or another.
There is also the question of what the term 'agent' actually means. Martin McGartland, who infiltrated the IRA in west Belfast, was an agent in the purest sense.
He joined the IRA at the request of his handlers and did exactly what he was told; it involved no switch of loyalties.
Several members of the IRA's internal security team, like Stakeknife, were double-agents. They were trusted by the IRA to frustrate Crown forces, but were 'doubled' by the intelligence services to spy on the IRA, instead.
After that, it gets more complicated. It is clear now that many of those who passed information to the authorities believed they were in charge of the relationship and didn't tell all they knew.
Many 'worked their passage' with the police, passing on this and that in return for favours, to settle grudges or to save their life. They may not have thought of themselves as agents at all, especially the loyalists.
Take the case of John McCoy, the Garda special branch officer in Monaghan who appeared on British military intelligence records under the code name 'Badger' in the 1970s.
I once interviewed Mr McCoy and it was clear that that was not how he saw the situation.
He had been tasked by his superiors to liaise with the RUC at a time when cross-border security co-operation was too politically sensitive to be done in the open.
His police contacts sometimes brought plainclothes soldiers with them and one of them, Bunny Dearsley, registered Mr McCoy as his agent, submitting source reports each time they met.
The Irish Barron Report looked into all this and accepted Mr McCoy's account. "All garda officers serving on the border were encouraged to cultivate intelligence contacts and to protect the anonymity of those contacts" it found, describing the relationship as "one of mutual exchange of information, rather than one of garda 'agent' and British Army 'handler'.
The same thing happened to Michael Foot, the Labour politician, who was registered as an agent by KGB officers working under diplomatic cover who befriended him and chatted with him about politics over lunch. In many cases, paramilitary 'agents' felt the police were colluding with them, or that individual officers were sympathetic friends. In some cases, they were correct in this assumption; in others, they got a rude awakening.
Raymond Gilmour, the supergrass, regarded himself as an honorary policeman - he was surprised when he couldn't get a police job in England after his cover was blown.
This is a murky picture, with many shades of darkness and few clear lights to navigate by.
However, if we are ever to learn the lessons of the Troubles we will have to accept that loyalties were not as simple as the prevailing narratives suggest.