This is a Swedish study, so what might be possible is using the population registry to contact siblings of the cancer patients to ask about traits like tattooing and then their health data would already be in Swedish system and linkable. This would control for a lot of the relevant confounders.
No, it would control for a lot of those factors. Things like impulsivity, sexuality, sensation-seeking etc. All very heritable or family-level. If you ever find any correlations reported for tattoos between family members, it's gonna look like everything else: steeply increasing with relatedness.
