The biggest discrepancy here concerns the climate effect of having children. For the reasons given, we think our estimate of the effect of having children is more accurate for people living in the EU or US states with strong climate policy, such as California, New York, as well as other states in the Northeast. Indeed, even outside the US states with strong climate policy, we think the estimate accounting for policy is much closer to the truth, since emissions per head are also declining at the national level, and climate policy is likely to strengthen across the US in the next few decades.
After taking into account policy effects, they find that the climate impact of having children is comparable to some other lifestyle choices such as living car-free. (I.e. it’s not the case that the climate impact of having children is orders of magnitude larger, as one might naively think w/o considering policy effects.)
You might also be interested in John Halstead’s and Johannes Ackva’s recent Climate & Lifestyle Report for Founders Pledge. They point out that taking into account policy effects can dramatically change the estimated climate impact of lifestyle choices, and on children specifically they say that:
After taking into account policy effects, they find that the climate impact of having children is comparable to some other lifestyle choices such as living car-free. (I.e. it’s not the case that the climate impact of having children is orders of magnitude larger, as one might naively think w/o considering policy effects.)
For more detail, see their section 3.