Great comment! For the iron - ageing link, we used two pieces of data: the effect of DNA on blood iron levels, and the effect of DNA on how long and healthy you live. We found the mutation that causes hereditary haemochromatosis had a large effect on early death (as expected). However, other genetic variations that increased blood iron levels also had a proportional effect on early death. So it looks like it is higher iron levels, not just haemochromatosis, that may accelerate the rate of ageing.