It's been debated for decades, remains a political hot potato and may be beyond the capacity of economic accounting. In favour of a significant contribution we have Britain, the leading 18th-century slave trader whose economy took off just as shipments were peaking: against it we have Portugal, the top trader overall, by the 19th century lagging despite the windfall of Brazilian gold, or Germany, which later surpassed Britain without having played a significant part in the trade.

For Britain, some estimates can be hazarded (note the choice of word). At its peak the trade seems to have involved under 5% of British shipping tonnage. Landings averaged about 28,000 slaves annually in the last decades before abolition: at £40 per head that represents a gross value of £1.1m a year, nearly a tenth of the official value of exports but under 1% of GDP. An alternative approach involves profits, which at £5 per slave would have totalled £11m over the whole of the 18th century, against a national wealth estimated at £1.4bn at its end.

But the trade in people is only a part of the story: plantation output in the British Caribbean is estimated at around £2m annually by the 1770s. In addition there is the stimulus to shipping, trade and production of goods to trade for slaves or to supply the colonies, plus the contribution of trade with other American territories to which Britain had supplied slaves, notably the US after independence. The last may be the most significant, for US cotton produced with slave labour proved a key element in Britain's industrialisation. The slave trade's indirect impact is likely to have far surpassed its immediate returns.

Much the same can be said of France, with still more lucrative Caribbean possessions until the loss of Haiti, but with a larger national economy into the 19th century and less than half of Britain's slave shipments. Nantes and Bordeaux prospered along with Bristol and later Liverpool, but the overall contribution of slaving and sugar was still less. Cotton never held the dominant position in French industry that it did for a time in Britain, but its role was again significant.

For the United States and the thirteen colonies that preceded it, direct participation in the slave trade was on a far smaller scale than the subsequent role of slave production: over the whole period of the trade North American traders shipped only a tenth as many slaves as their British counterparts, but cotton exports alone by 1860 approached $200m. But cotton production more than doubled in the next half-century despite Emancipation, indicating that not even slavery's pre-eminent product relied on the denial of all rights to millions.

The direct contribution of the trade to western European development has probably been widely overstated. Its legacy was greater but may be incalculable because of the difficulty of estimating indirect impacts and of determining how much of slave output might have occurred under a less abusive regime.