Mexico City — Is the the poorer two-thirds of the world simply too overcrowded for its own present and future good? Ten years ago at the world's first population conference in Bucharest, Romania, the third world itself, supported by the Communist bloc, argued passionately that the answer to overcrowding was not family-planning ''interference'' but long-term economic growth.
The United States and others urged that besides economic development, voluntary, sensitive, family planning was essential over the next decades to lower the number of children per family.
Today, delegates from 150 nations are gathering here in Mexico City for the second world conference - and the mood has changed.
Global population has jumped from 3.9 billion to 4.7 billion. The third world alone now contains 3.6 billion people and the United Nations says it will leap to 4.8 billion by the year 2000.
Famine faces more than 20 countries in Africa, where the world's fastest population growth is a major obstacle to economic gains.
One result: Asia and Latin America have adopted family planning and much of Africa at least sees the need for it.
''A broad consensus has emerged that the issue is not family planning or economic development, but family planning and development,'' says Raphael M. Salas, executive director of the United Nations Fund for Population Activities (UNFPA) and the man in charge of the conference.
But world population will keep growing for another 110 years, ''although at a slower pace,'' he told the Monitor.
Third-world countries, anxious for help, are encouraging voluntary groups financed by the UNFPA and private organizations such as the International Planned Parenthood Federation (IPPF) in London and the Population Crisis Committee in Washington, D.C.
Donor governments, led by the United States, now provide about $490 million a year to other governments and to voluntary groups, and the third world itself spends another $1.5 billion a year of its own funds.
Some success has been achieved. World growth rates fell in the decade to 1984 (although if China is excluded, third-world rates stayed almost unchanged.) China, India, Thailand, Indonesia, Sri Lanka, and other nations brought growth rates down with successful family planning programs.
However, two paradoxes are already generating much debate in Mexico City.
The first: The White House has switched signals on family-planning policy in what many critics, both inside and outside the Reagan administration, see as a move linked to President Reagan's need to appeal to the far right in his reelection campaign.
''The irony is that the US gives more money each year than any other country to promote family planning, but its delegation at Mexico City is about to deemphasize such planning and preach the virtues of free-market economic development instead,'' says Dr. Sharon L. Camp, vice-president of the Population Crisis Committee.
The White House has issued the controversial policy guidance to the US delegation and has chosen James Buckley as delegation chief. Mr. Buckley is the former Republican senator from New York, and is a staunch critic of family planning.
It is understood that Mr. Buckley's opening speech to the conference will blame third-world governments for mismanaging their economies, and will view population growth as an opportunity rather than a crisis.
Third-world countries say they will criticize such an approach with vehemence.
The policy defers to a vocal anti-family-planning lobby in Washington led by Mr. Buckley, by Senator Jesse Helms (R) of North Carolina, by Prof. Julian Simon of the University of Maryland, and others. Supported by many Roman Catholics, the lobby opposes any US funds supporting abortion in any way, or any coercion used to limit the size of families.
Its biggest success this year has been to persuade the White House to say that it will withhold US aid from any private group supporting programs considered to support abortion in any way.
This is specifically aimed at the IPPF in London, which received $11 million from the US in 1984 as part of a $55 million budget spread over 119 countries.
An IPPF spokesman says the organization spent $200,000 a year on programs in 10 countries where abortion is legally permitted. Population advocates hope that pressure will fade after the November presidential election in the United States is over.
The second paradox:
Despite the visible consensus that family planning is an essential short-term planning tool if economic growth is not to be swamped, the real value of donor aid is falling.
According to the US Agency for International Development, the world's biggest single donor, the $214 million it gave in 1983 was worth only $140.3 million in constant 1978 dollars. This means that US real aid has actually fallen since 1978, when the figure was $160.5 million.
In Mexico City, third-world nations will ask the US and other developed countries for more family-planning funds.
In an interview, the UNFPA's Salas said UN-channeled aid for population issues had amounted to only $1 billion in the last 10 years.
Mr. Salas estimated on the other hand that world spending on arms now amounted to $1.6 billion a day. This meant, he said, that the UN-linked population aid over 10 years would purchase arms for only 15 hours.
UN officials want to see donor aid doubled to about $1 billion a year, and the total of donor and third-world aid eventually reach some $5 billion annually.
Mr. Salas said he hopes that another result of the Mexico City meetings will be a ''renewed sense of urgency on population issues.''
He added: ''The aim of family planning programs is to allow third-world adults the kind of choice about how many children to have that Western world adults have long taken for granted.''
A recent preparatory conference urged delegates here: (1) to set specific targets to lower infant mortality, (2) to discuss the growth of cities, the status of women, and (3) to reinforce the need for effective, voluntary family planning.
To try and placate the Roman Catholic Church, which opposes artificial contraception, abortion, and sterilization, the delegates were also urged to give renewed attention to enhancing the role of the family.