Mexico City — At the heart of a wide gap between the United States and the third world here at the UN population conference lie two different views on human progress and behavior.
While presidential reelection politicking is widely seen behind the US position, the basic disagreements go deeper, many delegations say.
Among other effects, the split has put in immediate jeopardy $12 million in US family-planning aid. The money for the 103-nation International Planned Parenthood Federation of London will be cut off Oct. 1, the start of the 1985 US fiscal year.
The split also spotlighted divisions within the US delegation, which in turn reflected deep disagreements inside the US government. At the same time the conservative right-to-life lobby in the US is heartened. It says it will now work even harder to deny all US funds - $240 million this financial year - to other countries that permit abortion.
The two views of the world that conflict here involve economics and abortion.
Economics: The US, using one set of measurements (the number of children per family), sees grounds for optimism about slowing third-world population growth. It came here to offset what delegate Ben J. Wattenberg saw as a ''steady stream of fairly gloomy rhetoric'' from the United Nations, the World Bank, and otherwurces.
''We reject the notion that we are caught up in a global population crisis,'' said delegation leader and former US Sen. James Buckley in a press conference.
Complaining of being ''misunderstood,'' the delegation put together a set of US Census Bureau figures, based on 1982 UN estimates to support its case. The key figure, Mr. Buckley said, was that the number of children born to the average third-world family had fallen from 6.2 in 1950-55 to an estimated 4.1 in 1980-1985. This was halfway to an average of two children per family, or no increase in population at all, he said.
The figures showed other areas of progress: That between 1960 and 1982, third-world income per capita had risen, fewer babies were dying, people were living longer, more children were enrolled in school, more adults were able to read and write, and health care was better.
None of these areas were mentioned in Buckley's formal speech to the conference; other countries assumed the US was playing them down or ignoring them.
Mr. Wattenberg acknowledged ''big problems'' in the third world but said it was necessary to stress optimism to offset ''gloom'' and to persuade US voters to provide more aid.
The US position encouraged poorer countries to take US advice and adopt free-market incentives as the best way to boost incomes and lower birthrates. The US cited Hong Kong, South Korea, and members of the Association of Southeast Asian Nations as examples.
But the third world, supported by the World Bank, many other US officials, and private pro-family planning groups, saw the US view as doctrinaire and out of touch with the reality of their teeming villages and slums. Among the arguments of critics of the US position:
1. If China is excluded, the picture is much worse. The rates of growth in third-world nations have hardly fallen at all in the last decade.
2. While the US delegation focuses on population growth rates, nations such as Kenya, China, India, and Sri Lanka focus on actual numbers of people being added to the world each year: 80 million and rising every 12 months.
''Free enterprise works for richer countries, but we can't wait for its gains to catch up with our rapid population growth,'' said Mwai Kibaki, the vice-president of Kenya. ''It might take 150 years. We have to step up family planning now, and not deemphasize it.''
3. Reducing population growth must involve not only economic development but also voluntary family planning - a two-track solution.
4. The US free-market economic thesis is wrong over the short term. Centrally-planned economies such as China and Cuba have lowered birthrates dramatically through family planning. Countries whose incomes have not risen markedly, such as Indonesia and Thailand, have done the same.
Besides, third-world delegates said the examples of South Korea and Hong Kong were irrelevant and misleading. Korea had massive US investment after the Korean war and an active family-planning program. Hong Kong is a unique shop window and trading post for China.
Abortion: Why, critics ask, does the US intend to deny funds for abortion in other countries when abortion is legal in the US? The question is echoed by Democrats and some Republicans in Congress.
The Reagan administration is already deep in talks with the International Planned Parenthood Federation (IPPF), the largest private group of its kind, that will deny it almost one-quarter of its donor-provided budget unless it stops using its own non-US funds to support legal abortion-related programs in 10 countries.
But ''the US policy on abortion is just right,'' said Caroline Gerster, a Protestant physician from Phoenix, Ariz. Dr. Gerster is past president of the National Right to Life Committee in the US.
The American Life Lobby, headed by Judie Brown, says it will press for audits of UN and IPPF books to make certain no US funds are supporting abortion anywhere in the world.
Officials from the US Agency for International Development are braced for months of intense right-to-life pressure, unchecked by the White House. If President Reagan wins in November, they say, the pressure will be allowed to continue.
Meanwhile, Chinese delegates rejected any US effort to use financial aid to change the policies of other countries.
A third-world journalist asked the US delegation, ''How would the US feel if China were to say, 'Based on our success in lowering population rates, we think all other countries should adopt our economic system'?''
Alan L. Keyes, US representative to the UN Economic and Social Council and a US delegate here, replied that the question was hypothetical. Mr. Wattenberg asked what kind of economic system China had. When the journalist replied ''centrally planned,'' Wattenberg said Peking was adding ''some private elements into farming.''