Two kids: good. Dog and cat: fine. But Range Rover? That's a real no-no.

In the U.S., many population groups try to smooth over controversy, preferring to highlight areas of broad agreement, such as making birth control universally accessible, educating girls, and empowering women.

By contrast, the British group Population Matters (formerly Optimum Population Trust) tries to stir up controversy. It recently chided David and Victoria Beckham for adding a fourth child to their jet-setting brood, and the group has rankled some in the population movement by promoting PopOffsets, a program that lets you “offset” your family’s carbon footprint by funding family planning elsewhere.

I talked to Roger Martin, chair of Population Matters, about the group’s approach and the impending milestone of 7 billion. He shared some T-shirt-worthy slogans — “It’s no use reducing your carbon footprint if you keep increasing the number of feet,” and “Any resource shortage is in part a population longage” — and some harsh words for environmental groups that ignore population growth.

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Q. Population Matters criticized Posh and Becks for having four kids. Do you think you risk turning people off by being so combative?

A. For those who are aware of the implications of having three or more children, and who have the option not to, it is irresponsible, or it at least raises moral questions they should think about. Frankly, we just try to get people to focus on the fact that there are ethical implications [PDF] in choosing to have a third and subsequent children.

Q. Do you think population is getting more attention these days?

A. I’m confident that the mad taboo on discussing it is crumbling. It was bound to crumble because it was so bloody stupid, so contrary to self-evident fact. It’s crumbling rather slowly and some of it’s still there. I’d love it to crumble completely so that everybody was able, free from hysteria and finger-pointing and all that rubbish, to discuss the more interesting question: not, “Is population growth a problem?” but, “What are the best programs for stabilizing numbers as quickly as possible in an acceptable way that does not infringe on civil liberties?” I’m always being put on the BBC to argue against some idiot who says, “No, it’s not a problem at all.”

Roger Martin, chair of population matters.

Q. Why do so many environmental groups avoid talking about population?

A. It’s monstrous. The NGOs in my view are telling a silent lie every time they put out a statement on some environmental problem where they know that every additional person makes it harder and ultimately impossible to solve.

I suspect, having been through the process myself as head of an environmental NGO, that it’s to do with professionalization. When I started with a staff of three, I was a hard-hitting campaigner with a lot of volunteers. By the time I finished 10 years later with a staff of 35, my priority concern, if I’m honest, had switched from preserving the wildlife of Somerset to keeping my staff and myself in work. So not upsetting funders became more and more important, and it made us ever more toothless watchdogs. And I think that’s happened at the Sierra Club and many others. They’ve become part of the establishment, they don’t rock boats.

Q. Does Population Matters want governments to enact policies to promote smaller families, or are you just trying to convince individuals to act on their own to have fewer kids?

A. We don’t want policies in the form of legislation; we’re against coercive population policy, and indeed it would never work in a democracy. We want governments to state the problem up front, recognize that all our environmental problems get harder and ultimately impossible to solve with ever more people, and that it’s in the national interest therefore to stabilize our numbers and then reduce them by voluntary means to a sustainable level as soon as possible. And [we want governments] to invest money in culture-shifting public education programs of the kind that they’ve run successfully in recent decades for drunk driving, smoking, sex without condoms, and so on.

Q. Does Population Matters take a stance on immigration policy?

A. Yes. Half our population growth [in the U.K.] is due to migration, so [we advocate] balanced migration to stabilize that — no more in than out. We find this position uncomfortable because it puts us in unpleasant company, but we are forced into it by intellectual honesty. (And we ignore, by the way, intra-European Union flows for that purpose because we’re already legally obligated to permit free movement around the E.U.)

There is no possibility of getting a global [population] policy adopted. It’s hard enough to get global carbon policies adopted. So insofar as there will be successful population policies — non-coercive, good family planning, all the rest of it — they have to be conducted at the national level.

Q. Do you mostly concentrate on British people or Europeans rather than people in developing countries?

A. It’s 50-50. In developing countries, there are 215 million women with an unmet need for contraception. We should fund the programs to provide [family-planning] services and female education and empowerment.

It’s just as important that us rich people, who emit 22 times more carbon per head than a Malawian, should take responsibility and stabilize our numbers.

Q. Are there countries that have done a particularly good job of dealing with population pressures?

A. The two best are Thailand and Iran, which have both achieved very similar reductions in fertility through entirely voluntary means: you provide the services, you make them affordable, preferably free, good range of options so women can use the contraception of their choice, you get the NGOs on board, you have consistent messaging, and you invest in programs to liberate women and empower women.

Read more on population. Check out our series

7 Billion: What to expect when you’re expanding
Q. What is the significance of 7 billion?

A. Objectively, that 7 billionth kid is no more significant than the one before and the one after, but it does rub in the fact that the U.N. projects the population in 2050 will be somewhere in the range of 8.1 to 10.6 billion. If you ask yourself which is easier to feed, 8.1 billion or 10.6 billion, which is easier to supply with water, which will emit less carbon, which will have less impact, which will deplete oil reserves and other mineral reserves faster, the answer doesn’t require any research at all, it’s straight from the university of the bleeding obvious. For everyone’s sake, the sooner we can stabilize to as near as possible to 8.1, our kids will have a vastly greater chance of a halfway decent life.

On a finite planet, we know for a fact that indefinite growth in anything physical is physically impossible. So physical consumption of resources per person and the number of consumers will quite definitely stop at some point. It will either be sooner, the nice way, through fewer births, or later, the nasty way, through more deaths. But there is no third alternative.

Watch Martin talk about population issues on BBC Hardtalk.