I don't much like the sight of a newspaper editor being asked by a parliamentary committee whether he loves his country. In a disquieting mental image, Keith Vaz, the fruity-voiced chairman of the Commons home affairs select committee, morphed momentarily and implausibly into the foul-mouthed Senator Joe McCarthy. Still, I couldn't fault Alan Rusbridger's reply: "I'm slightly surprised to be asked the question but, yes, we are patriots, and one of the things we are patriotic about is the nature of democracy, the nature of a free press."

Quite. This country has been distinguished for more than three centuries by uncensored newspapers. Here are some more defining traits: equality before the law, without regard for birth, wealth or ancestry; the mass franchise; universal education; jury trials; regular elections. There are few places on Earth of which lefties should be prouder.

Even as I type, I sense thunder gathering in the comment thread. It's one thing for the Guardian editor to say he is a patriot; quite another for me to compare Britain favourably to other countries. What about slavery and colonialism and the House of Lords and Peterloo and Croke Park and Amritsar?

Like all nations, we have sometimes behaved shabbily. But I stand by my claim: no other civilisation has been so secure a repository for human freedom. Our indigenous radical tradition has deep roots: roots that stretch back through the suffragettes and the Chartists; back through John Wilkes and Thomas Paine; back, arguably, even through the Levellers to the Lollards. So why has patriotism come to be associated largely with the political right?

It won't do to say that national feeling is intrinsically conservative. Plenty of nationalists around the world have been self-conscious revolutionaries. The leaders of the 1848 risings across Europe were overwhelmingly men of the left. When they were defeated, and the monarchical-clerical order was restored, most fled to London, trusting to our country's sympathy with freedom.

Even today, plenty of leftist movements define themselves in terms of national resistance against foreign-backed oligarchies. Think, for example, of the Bolivarian regimes in my native Latin America. But here's the odd thing. I have plenty of Labour-voting friends who are happy to cheer Venezuelan nationalism, but who would be mortified to be called British nationalists.

Why? Well, at the risk of annoying Guardian readers even more, it may be to do with how our brains are wired. In his seminal 2012 book, The Righteous Mind, the psychologist Jonathan Haidt set out to explore differences in left- and rightwing brain circuitry. Progressives and conservatives alike lean, unconsciously, towards particular conclusions, and then scrabble around to rationalise those conclusions to themselves. The key thing is to find what makes them lean, not what ex post facto reasoning they use.

Leftwingers, Haidt discovered, lean overwhelmingly on the basis of concern for the downtrodden. That sentiment is felt by conservatives too, but in conservative brains it's balanced by other factors: concern for liberty, for loyalty, for reciprocity. The further left you are, the more your concern for the underdog crowds out everything else, leading you to overlook inconsistencies. You might, for example, argue for immigration and multiculturalism in the UK, but not in the Amazon. You might demand equality before the law and, at the same time, gender quotas. You might support Israel when it fights the British, but not when it fights the Palestinians.

Which brings us to a peculiarity of British history. Put simply, there are not many occasions when we can plausibly be cast as the underdogs. Even the moments that left and right should uncomplicatedly cheer – the fact, for example, that we diverted ships and men to stamp out the slave trade even while struggling against Napoleon – are somehow tainted by our technological supremacy.

Slavery, in fact, illustrates the point rather neatly. It was practised on every continent and archipelago, from the agrarian revolution onwards. Every human on the planet is descended from both slaves and slave owners. What makes Britain unusual is not that we engaged in the disgusting trade, but that we eliminated it. Our political institutions led us, earlier than many, to the conclusion that freedom was the highest virtue.

As Billy Bragg, a rare champion of leftist nationalism, puts it: "If there is a single trait in our character that has set us apart from other nations, it is our determination to limit the authority of those who rule over us".

It is easy – facile, indeed – to complain when a past generation offends modern values. As Herbert Butterfield put it in the 1930s: "The study of the past with one eye upon the present is the essence of what we mean by the word 'unhistorical'."

The fact that Thomas Jefferson owned slaves doesn't devalue what he wrote. The fact that Winston Churchill wanted to use poison gas on rebellious tribes in India doesn't diminish his war against fascism.

The contrast that matters is not with later generations, but with contemporaries. Ours is the culture that gave the world Magna Carta, the Bill of Rights and parliamentary democracy; that fought to liberate Europe from Nazism; that raised the prosperity of the ordinary citizen to a level previously unimagined. Who did more? Austria? Persia? Ethiopia? Against whom are we being so harshly judged?

Daniel Hannan is the author of How We Invented Freedom & Why it Matters published by Head of Zeus