MY friend Russell used to call it “the unrequited hate affair”.
While living in London 15 years ago, he took in a new flatmate fresh off the train from Glasgow.
Flame-haired and proudly Scottish, she expected to be on the receiving end of a certain level of hostility in the English capital. After all, she reasoned, the average Sassenach in Glasgow takes a fair amount of abuse.
But she was surprised to find that not a single Londoner, on hearing her Caledonian accent, expressed any kind of hostility.
We see a similar phenomenon in the world of sport.
Many Scots will cheer any opponent of England in an international competition, hoping to see the Auld Enemy get a black eye.
Most people South of the border, considering Scots to be virtually family, would never dream of returning the insult.
Supporting a foreign country against fellow Brits makes about as much sense to many English people as hoping your brother mucks up his job interview or wishing your cousin fails her driving test.
Looking at all this, Russell concluded that the Scots had an unrequited hate affair with the English.
They loathed their Southern neighbours but received nothing in return except affection or, worse still, indifference.
When I first heard Russell explain his theory I thought it was an astute characterisation of the relationship between England and Scotland.
But somewhere along the line in the last 10 years, his observation has become out of date.
There was a point in the past decade when the hate affair became requited, when Loathing Lane became a two-way street.
Many English people have grown increasingly resentful as the Scots have enjoyed the fruits of devolution by opting out of tuition fees and prescription charges.
But, as with the debate about public sector pensions, justified anger is directed at the wrong target.
Rather than demanding to know why the Scots get free prescriptions, perhaps English people should ask why they do not.
But it is easier to ignore this issue and instead portray people North of the border as “subsidy junkies” who live the good life off English money.
However, this insult is based on the strange idea that oil under Scottish water magically becomes British when it’s brought to the surface.
It’s rather like pickpocketing your dining companion and then calling him a sponger for not paying his half of the bill.
But the main reason that many in England have turned against the Scots comes in the shape of the Member of Parliament for Kirkcaldy and Cowdenbeath.
Gordon Brown was a spectacularly unpopular prime minister who led the country during a very difficult period.
His opponents were not slow to use his Scottishness against him, pushing against the open door of English hostility to all things Caledonian.
A few years ago a Conservative politician told me Gordon Brown was “a scheming Scot”, a phrase which he then repeated and encouraged me to quote.
In the end I decided against using it. But it was interesting that this politician had hoped this comment would end up in the public domain.
He had obviously concluded that there was more to be gained than lost by drawing attention to Mr Brown’s nationality. I think he was right.
Into this increasingly acrimonious atmosphere between the two countries comes the prospect of a vote on Scottish independence.
David Cameron piled into the debate last week, demanding a vote next year rather than 2014 and trying to determine the question to appear on the ballot paper.
Perhaps the Prime Minister believes Scots enjoy being told what to do by an upper-class Englishman. Or maybe Mr Cameron thinks the average English voter will enjoy wee Jock McSubsidy being ordered to put up or shut up.
It’s far from clear whether put up would beat shut up in a straight fight.
A poll this week showed that 39% of voters North of the border now back independence. The Scottish National Party hopes two-and-a-bit years of campaigning will push this over the 50% mark.
I am not so sure. I have long believed that, if push ever came to shove, the Scots would not quite have the guts to go it alone.
Independence would be a huge change, full of both potential and risk.
By contrast, voting to remain in the UK is the safe option – and very far from the worst thing in the world for Scotland.
When it comes to it, I think the Scots will take the lesser of two risks and decide to stay in the union.
But, given the deteriorating relations across Hadrian’s Wall, this would still leave one question unanswered.
If the Scots don’t vote to jump, will the English vote to push them?