The study found reduction in life expectancy to be 6.78 years for males and 8.64 years for females. I wonder what could contribute to that difference.
It's not clear to me that there is a difference. It looks like the confidence intervals overlap.
ADHD is diagnosed roughly twice as often in men than women, and when diagnosed in women it is much more likely to be of primarily inattentive rather than hyperactive type compared to men. Many suspect ADHD may not actually be rarer in women, but just that the inattentive type is harder to diagnose, and more often never diagnosed- mostly because it is less disruptive to others in school settings.
I speculate that the women that are diagnosed are much more enriched for severe inattentive type ADHD, and I also speculate that inattentive type has a shorter life expectancy than hyperactive type. Just judging from myself, and other adults I know with ADHD: the hyperactive type ones tend to remain really physically active even into old age- unusually so, and the inattentive type tend to have a lot more depression, anxiety, and stress.
78.6 years for males, so 6.78 years is 8.6% of the average life expectancy
82.6 years for females, so 8.64 years is 10.4% of the average life expectancy
I think this is a relatively small difference in effect between the two, but I can't say for certain.