Drawing lines: how the public defines "Serious" genetic conditions for reproductive testing

Prior normative and empirical research has identified a gap between how patients and the public conceptualize "serious" conditions eligible for preimplantation genetic testing for monogenic disorders (PGT-M) and how seriousness is defined. Japan represents a highly restrictive regulatory context shaped by its historical experience with eugenics. Although PGT-M has been available since 1998, its use has been tightly limited, with the first birth occurring six years later and approximately 36 conditions approved to date. This study examines how Japanese adults of reproductive age understand "seriousness" in genetic disease and how they view PGT-M in relation to concerns about eugenic selection (inochi no senbetsu, "sorting of lives"). We conducted a cross-sectional online survey of 458 adults aged 18-42 years. Measures included perceived seriousness across clinical parameters (penetrance, survival, daily life impact), willingness to use PGT-M, perceived barriers to access, and attitudes toward eugenic selection. Awareness of PGT-M was low (12%), yet 44% of participants would consider its use. Participants defined seriousness more broadly than current Japanese policy permits: most rated highly penetrant or childhood-lethal conditions as serious, and over half also included conditions associated with mild to moderate functional limitations. Participants distinguished disease prevention from enhancement. Economic burden was the most frequently cited barrier (70%). Many participants who expressed ethical discomfort with eugenic selection nonetheless reported willingness to consider PGT-M. These findings reveal a substantial gap between policy, public reasoning, and reproductive preferences, with implications for European debates on seriousness and proportionality in reproductive genetic testing.