The idea behind Copenhagen Consensus 2012 is to prioritize some of the world’s most important spending, with the latest economic analysis providing us with an input. In a series of articles, I am drawing on new research papers that present the costs and benefits of the smartest ways to respond to global challenges. Each article has a poll, and Slate readers can vote on the investments that they believe should be the highest priorities–along with those that should not. You can read more about the rationale behind the project here.

Over the course of the series, we will see how readers rank responses to a range of challenges, and at the end we will identify the investments that Slate readers think should be the highest priority. We will be able to contrast these with the findings of a panel of Nobel laureate economists.

Yesterday’s piece prompted some feisty debate and discussion between Slate readers. The concept of overpopulation has been a hot topic since the series began, and some readers believe that changing demographics underpin all of the other challenges that we have been looking at, like malnutrition and chronic and infectious disease. (I’d make the point that, as we saw in the article on the hunger research, there is actually more than enough food being produced for everybody; we’re just not very good at distributing it to those in need.)

There also seem to be some Malthusians among readers, with a very pessimistic view of humanity’s chances, especially because of a fear that we are “overconsuming” and will run out of natural resources. (Next week, the research on climate change will give us a chance to explore and discuss the different ways we can make a shift to green energy sources). Other readers worry that without population growth, society will die.

The problem of aging populations was raised by some readers. Changing demographics are certainly going to pose major challenges—this U.K. parliamentary publication points to some of the looming problems there. Responding to those challenges will, arguably, be even more complex than trying to assist with family planning needs in the fragile countries whose populations are growing very rapidly.

One point from yesterday bears repeating, and that is that the research focuses on filling an “unmet need”: reaching those women who want to stop having children (or delay their next childbirth) but are not on contraception. This should be the morally least objectionable issue in the population discussion, since we’re really just talking about providing what is already demanded by individuals.

Now, let’s turn to how Slate readers voted in yesterday’s poll, and how that result fits into the results so far:

Remember: You can still go back and vote in all of the various polls today, and you can vote today on options to respond to natural disasters; we’ll be finalizing the Slate readers’ list at the end of the series.

And there’s been an upset at the top! Family-planning assistance is now the single investment that Slate readers think should be prioritized by policymakers and philanthropists. And another change at the top: based on your latest votes, tobacco taxation has slipped underneath micronutrient provision as the second-leading priority. It’ll be interesting to see how this picture changes in the second-half of our series, beginning Monday. Keep the votes—and comments—coming!