If a financially secure, well-educated couple decide to have an extra child, is that likely to be a good or a bad thing for the world, on net? Many (especially environmentalists) assume the answer is "bad", on grounds of over-population: More people = more consumption = more pollution (including greenhouse gas emissions). But that's not all there is to consider. For one thing, additional good lives are plausibly of non-instrumental value. The inherent value of the new person's life is nothing to sneeze at. Moreover: productive, law-abiding citizens create significant positive externalities. Think of all the ways your life is improved by the labour of others, from farmers to car mechanics to entrepreneurs and scientific researchers. People are, as Julian Simon famously wrote, the "ultimate resource". More people = more ideas and productive capacity = more solutions to whatever problems beset us.
Consider climate change. A certain degree of "techno-optimism" here is, I think, rationally mandatory. Between improvements in renewable energy technology (esp. the dropping price of solar panels), electric self-driving cars (which could reduce our overall ghg emissions by 16%), vat-grown meat and actually-tasty meat substitutes to replace the wasteful livestock sector, etc. etc., it's unquestionably the case that technological advances will improve our ability to reduce harmful emissions (and that's not even to mention more speculative means for mitigating climate change, e.g. geoengineering -- which, though we obviously don't want to rely upon it, is surely an important field of research in case our climate models turn out to be over-optimistic, and truly catastrophic climate change appears on the horizon).
(None of this, I should clarify, is to engage in any kind of techno-optimist "denialism" which says we needn't worry about climate change at all because future technologies will magically save us. On the contrary, I'm assuming that we do need to worry about climate change, and invest in improving our capacity to deal with it, and increasing population -- especially amongst the most privileged demographics -- is a way to do that.)
So it's far from obvious that additional people (again, especially those raised in ideal circumstances, who are presumably most likely to go on to do groundbreaking work) are bad for the world on net. If anything, I think it's much more plausibly good on net.* But it's an open empirical question, where we should ask the experts. Any economists in the audience? Is there a consensus amongst demographic economists regarding whether the marginal value of additional population in developed societies is positive or negative on net, taking into account environmental externalities?
*: One reason for skepticism regarding the costs of additional lives is that (according to GWWC) the WHO estimates the health cost of carbon emissions to be 1 DALY per 5000 tons of CO2-equivalent, while the average American emits 20 tons of CO2-eq per year, suggesting a lifetime cost of about 1/3 of a DALY per additional American (half that for a Brit). Moreover, (again, according to GWWC's research) it takes less than $1/ton to offset carbon via donations to Cool Earth. So there's little marginal harm being done here, and it's very easy to compensate. (One can do many times more good, however, by donating to GiveWell's top charities, whose cost-effectiveness is on the order of $50/DALY, rather than nearly $5000/DALY.)
In short, if these estimates are right (and assuming that climate change constitutes at least half the total environmental damage done by additional lives), a mere $20 donated to GiveWell's top charities will more than offset the gross harm to human beings** done by having an additional child in Britain (double it for Americans). That already gets them into the green. Any additional value they happen to produce for the world throughout their life is then a pure bonus.
**: Of course, given the harms to non-human animals of factory farming, you should also raise your kids to be vegetarian.
[This post was inspired by Sarah Conly's very interesting recent colloquium talk, defending the view that we don't have a right to more than one child.]