AT CANCUN, President Reagan declined to support Global Negotiations--in the elevated sense that requires capital letters. But Global Negotiations on the world's economy were earnestly desired by most of the governments represented there, most forcefully by those from the poorest countries with claims of social justice to press. Was Mr. Reagan being churlish? Was he wrong?

No. He was speaking for reality. Let's put it this way: if a hypothetical American president intended to do absolutely nothing for economic development in the poor countries, and if there were no limits to his cynicism, he could get rid of the whole issue by cheerfully supporting Global Negotiations in the United Nations. With that, a long list of urgent economic questions would vanish into a decade of vaporous resolutions, drafted carefully to avoid bruising any participant's real interests.

Oil prices are central to economic growth. Since the OPEC countries themselves have been unable to agree on oil prices for the past two years, how would you rate the chances of worldwide agreement at the United Naitons? As for wheat, everybody knows that it would be intelligent to set up an international reserve against recurring bad harvests. But after years of desultory talks, there's been no agreement regarding who's to pay for it. Poor countries want, with good reason, international agreements to stabilize prices of their exports. But past experience, as in the coffee and tin agreements, has not been promising. It's frustration with these agreements that leads some of the poor countries to hope--mistakenly--that adding more products and more complexity to the present system will force it to work more effectively.

Poor countries' access to credit is crucial, but any discussion of it immediately splits the governments seeking suspension of old debts from the governments seeking new loans. Poor countries' access to rich countries' markets, free of harassment by tariffs and quotas, is also crucial. But you will note that some of the Europeans most enthusiastically supporting Global Negotiations at Cancun are simultaneously working diligently in real negotiations at Geneva to keep poor countries' textiles out of the Common Market.

It would have been helpful if Mr. Reagan had had a little more to say about American obligations, both moral and economic, to less fortunate people. But on tactics he was making a sensible point. Perhaps there are questions best resolved in worldwide discussions, although it's hard to think of many plausible examples. More frequently, they can better be left to the two or three governments immediately concerned. Or they can be taken to the specialized international agencies on trade and finance. The poor countries--that is, all but the two dozen richest--essentially are pursuing a greater share of the world's economic power. Mr. Reagan is telling them that they won't get it through U.N. resolutions and, meanwhile, they all might more usefully work together for their common benefit.

With growth, the distribution of the world's wealth changes and, even among the poorest countries, the conditions of life improve. Americans have always considered themselves a well-fed and healthy people. Bangladesh, in contrast, is the world's metaphor for famished destitution. Yet the life expectancy of an infant born today in Bangladesh is longer, at 49 years, than that of an American born as recently as the year 1900. The talks by the sea at Cancun were useful because the subject was a kind of progress that continues to be visible and demonstrable.