Mexico City — At the world's biggest population conference for a decade, attended by about 140 countries, the Reagan administration finds itself the odd man out - isolated from the third world.
The result is the biggest single controversy of the conference (Aug. 6-13), and it is filling corridors and meeting rooms with puzzlement and concern.
The heart of the issue is a 15-page White House policy document issued last month.
The document, written to guide the US delegation here, breaks with traditional US policy in several ways.
Its thrust is that overcrowding and resource shortages are best overcome less by family planning and more by free-market economic policies. Pointedly, it blames third world governments for mismanaging their economies.
It also states a new US policy on restricting funds for family planning.
Since 1974, US law has forbidden government-to-government aid for any programs that include abortion or abortion-related activities. The new policy goes further by cutting off US aid to private groups which support programs abroad that are in any way related to abortion.
The document has left third world nations puzzled because the US is still committed by the US Foreign Assistance Act to support voluntary family planning. The US has given more funds for family planning aid in general than any other world donor.
The policy statement also conflicts with the Democratic majority in the House of Representatives. An Appropriations subcommittee has just voted to increase family planning aid funds for 1985 by $50 million to $290 million.
Critics of the Reagan administration say the new line is a political ploy in a presidential election year, aimed at appeasing the far-right wing on the eve of the GOP convention in Dallas.
The document does not rule out family planning entirely over the short term, but reiterates that ''attempts to use abortion, involuntary sterilization, or other coercive measures in family planning ... be shunned.''
Third world delegates are asking who speaks for the United States:
* James Buckley, the White House-appointed US delegation chief and former Republican Senator from New York, who is perceived here as part of a lobby close to Mr. Reagan that is anti-abortion, anti-family planning, and largely Roman Catholic?
* Or Democrats in Congress, plus the State Department and the Agency for International Development (AID), who support existing policy?
Third world delegates are also faced by a concerted campaign here by private US lobby groups to oppose the position of the US delegation.
In an interview, Werner Fornos, president of the Population Institute in Washington DC, said he was spreading the word that the White House policy ''is rhetoric which conflicts with US law, and which Congress will not carry out.''
And, on Aug. 3, the Rockefeller Foundation released here a Gallup Poll it commissioned. Of 1042 Americans polled between July 9-15 this year, 82.9 per cent said economic growth was hurt by high rates of population growth.
Opinion was more divided on specific aid for family planning. Almost 57 percent favored US aid to ''reduce population growth,'' 32 percent disapproved and the remainder did not answer or did not know.
Thirty five percent thought population control should be a condition of development assistance, but 50 percent did not. The rest had no opinion.
Opinions on abortion issues were also mixed. About 34 percent said the US should support both family planning and abortions abroad. About 28 percent approved and 24 percent opposed US aid to countries where abortion was legal.
Third world delegates contacted in recent weeks told this newspaper that the poorer two-thirds of the world was likely, in the long run, to disregard the new White House line.
One senior United Nations official privately called the Reagan line ''out of touch with the reality of overcrowded villages and slums around the world and the threat to health, jobs and security.''
The policy ''could result in more population growth, and even more abortions, '' he added.
''You ask us to promote free enterprise, but when we try to sell textiles to you, you slap on high tariffs,'' commented Indonesian Minister for Population and Environment Emil Salim.
''Besides, rapid population growth is a security issue for the US,'' Mr. Salim said. ''How can you rest comfortably knowing that so many countries have too many people competing for food, water, land and jobs?''
And a leading Mexican official commented: ''How can the US, which has pushed us for a decade into controlling our population growth, send a delegation to our own capital city arguing this kind of case?''
The White House document agrees that there has been an ''unprecedented'' spurt in world population in recent years, but calls it an evidence of ''human progress'' - a triumph of better health care, emergency famine relief, and improvements in agriculture, engineering and education.
Needed now, the document says, are economic freedom and reforms in the third world which would deal with the population situation.
The Reagan White House believes that third-world governments should stop imposing price controls on food and allow farmers to earn higher income from what they grow. Such policies, it contends, would help expand resources in the long-run to meet the needs of rising population. Also rising incomes from freer economies would encourage couples to have fewer children, the policy says.
The White House document, in tightening controls over AID assistance to private groups, seeks to ban any US funds from programs which ''perform or actively promote abortion as a method of family planning in other nations.''
This threatens $11 million of US assistance to the International Planned Parenthood Federation in London, which supports programs in 10 countries where abortion is legal.
In June, the White House tried to restrict US family planning aid even further.
It issued a policy document for the United States delegation attending the population conference in Mexico City which would have ended such US government support to all other governments which permitted legal abortions.
This would have threatened US family planning aid to India, among other countries. An immediate outcry in Congress and among private family planning groups eventually forced the White House to modify its stand. It left intact government-to-government aid and took aim instead at private groups receiving US funds and assisting programs including abortion.