A common thought pattern in Environmental Studies students is that (un)sustainability is, at its heart, a population problem. This thinking hangs, in most cases, on the IPAT equation: (environmental) Impact = Population * Affluence * Technology.
Part of the mindset that unsustainability is a population problem probably comes from the fact that the P multiplier is listed first. Intuitively, that must mean it's the most important, right?
Part of that mindset comes from the fact that everybody knows global population is expanding, so it's easy to conceive that environmental impact must be expanding as a result.
Part of it, I suspect, is evidenced by the fact that almost every time students put forth population controls as a potential solution to sustainability problems, they're talking about population controls somewhere else. Not here. In the developing or underdeveloped parts of the world where population is expanding most rapidly. Over there.
But, no matter how much the IPAT formulation tends to focus minds on the "P" term, population isn't the problem. In fact, if you take IPAT as a strict mathematical formula, the P multiplier has precisely zero effect. To understand this, forget technology for the moment. Look just at population and affluence. If technology doesn't change, impact is the product of population times affluence. But what's "affluence"? Turns out, it's an expression of resource consumption per capita. Per capita. Averaged out. Total consumption divided by the number of people. Divided by the population. So "P times A" turns out to mean population times consumption divided by population. The P term washes out. It has no impact at all. Add a person without affecting total consumption, and impact is unchanged. Subtract a person, same thing. It's not the number of people; it's the total amount of consumption.
So why's the P term even there? My inner cynic tells me that having P as the most prominent factor makes IPAT less uncomfortable for many students. Tying impact to some combination of population, affluence and technology is less threatening than the simpler formulation that Impact = Consumption. For students who have grown up in a consumer economy, who've never known anything but a consumerist society, who aspire at some level (whether they're ecologically sensitive or not) to being able to afford ever bigger and better forms of consumption, I=C is anathema. I=PAT is far more acceptable. Even if (at least as internalized by many students) it turns out to be misleading.