A shudder, whether of revulsion or mere embarrassment, tends to precede rational reflection on the topic of immigration, and as often as not replaces it entirely. Xenophobes fear the alien in their midst: the smell of strange cuisines, the presence of other races and the outlandish appearance of foreign garb. Liberals, on the other hand, bristle in reaction to these very same prejudices – which, to be fair, make even many Conservatives squirm. Ukip supporters, described by David Cameron as "closet racists", experience a kind of double shudder at both immigrants and domestic bigots. Ukip voters, so the jibe runs, would like to vote BNP, but shop at M&S; a genteel bashfulness and net-curtain propriety constraining cruder expressions of anti-immigrant sentiment.

The economist Paul Collier aims to introduce a measure of nuance – perhaps, as he recognises, unwanted nuance – into this "toxic" sector of public debate, where "high emotion" inspires "fundamentalists" on both sides of the argument. Liberals, however benign their intentions, turn out to be no less emotive in their predetermined approach to immigration than the small-minded racists and nationalists from whom they recoil. Immigration, Collier contends, has been out of bounds for liberal thinkers: "The only permissible opinion has been to bemoan popular antipathy to it." Postcolonial guilt about historic injustices tends to shape responses to current migration policy, while stifling consideration of wider problems of global poverty.

Collier is the author of a previous book on the "Bottom Billion", and he asks the pertinent question whether migration to the west harms the economic prospects of the very poorest people on the planet. Migration is, Collier argues, not a matter simply of us and them – the host society and the migrants it receives – but a triangular set of relationships, which also includes those "left behind" in the impoverished societies from which migration drains the bright, the ambitious, the entrepreneurial and often – since migration is not cheap – the wealthy. Yet this triangle is far from equilateral, for the immigration policies of rich countries "inadvertently" set emigration rates for the world's poorest societies.

There are of course counterarguments to the assumption that emigration harms poor countries. The remittances sent home by migrants also contribute to the wellbeing of those left behind, and, Collier concludes, at present more than compensate for the brain drain. At present estimates, global remittances amounted to $400bn in 2012 – four times the amount of international aid transfers and equivalent to the scale of inward investment in poorer economies. Besides, other indirect benefits flow from emigration. The exposure of migrants to the culture and institutions of advanced democracies can also feed back as an indirect "brain gain" to the country of origin in the form of pressure for improved standards of governance.

Nevertheless, there are further subtleties – and grotesque ironies – that need to be addressed. Education expenditure in low-income countries with high rates of out-migration – the training of our workers for free, in other words – is unmasked as "a hidden form of aid from the poor to the rich". Why should developing countries bother to subsidise the education of potential emigrants? The case of South Sudan adds a further troubling twist. Skilled emigrants from the war-torn region have now returned, but leave behind their families in the west, to whom they send remittances. Bizarrely, Collier finds that "one of the poorest countries in the world is making net remittances to some of the richest".

Notwithstanding spectacularly perverse outcomes of this kind, migration – up to a point – is good for all concerned, Collier reckons. It certainly provides economic returns to the migrants themselves and helps, on balance, the societies from which they came. Although there are some social costs, it does not – contrary to the claims of extremists – drive down wages for indigenous workers in the host country. The only group that suffers economically from the competition posed by immigrant workers is the previous wave of immigrants. "Moderate migration," Collier concludes, "is modestly advantageous."

The big danger lies in runaway migration. Collier identifies an "acceleration principle" intrinsic to the migration process. While massive emigration does not, it seems, drive up incomes in countries of origin, a growing diaspora of migrants in the host country eases the practicalities of migration, and adds further momentum to the trend. Runaway migration threatens to outpace the absorption of the diaspora in the home country – with worrying social consequences – and will eventually impose the much-touted downward pressure on indigenous wages.

Worse still, the country of origin will also be denuded of its human capital. Haiti, for example, has already lost about 85% of its educated workers. Very poor countries which also happen to be small are acutely vulnerable to the effects of high emigration.

Collier parts company with those economists who champion open-doors migration on the grounds that it maximises "global utility"; that, because the gainers gain so much more than the losers lose, the losers' interests can simply be written off. He argues instead that nations are "legitimate moral units" that must be taken seriously – whether small poor nations, with their intractable and worsening problems, or successful modernised nations. Post-national visionaries forget that it is "the fruits of successful nationhood" that attract migrants in the first place.

Yet mass immigration jeopardises feelings of national solidarity, which in turn seems to drive western societies in a dangerously rightward direction. What Collier terms "mutual regard" – a shared sympathy with one's fellow citizens – is an essential precondition of redistributive taxation. Robert Putnam, the leading American sociologist, has shown that the higher the proportion of immigrants in a community, the lower the levels of trust not only between indigenous and immigrant populations, but also within these groups.

Fraternity, it transpires, is no mere rhetorical flourish, but a necessary adjunct of liberty and equality. Collier asks us to consider transnational forms of redistributive taxation. The EU transfers less than 1% of European income between member states. The Germans, exposed to decades of Europhile cheerleading, still remain reluctant to dig into their pockets to support their fellow Eurocitizens in Greece. Think of international aid as a kind of global mechanism for redistributive taxation: in most major countries it falls below 0.7% of income. Notwithstanding the horrors of ethnic nationalism, nations remain "virtually our only systems for providing public goods".

Collier's book is humane and tinged with poignancy. He writes openly of his own descent from migrants. His grandfather emigrated from rural poverty in the German village of Ernsbach to the mill-town of Bradford, but was caught up in the anti-German xenophobia of the first world war when their shop was attacked by a mob. Collier's keen empathy with both migrants and those left behind informs a cautious prescription – a door neither open nor closed, "but one that is ajar".

• Colin Kidd's books include British Identities Before Nationalism.