For a study, the researchers aimed to identify autism spectrum disorder (ASD) biomarkers, which was an essential priority for understanding etiology, facilitating early diagnosis, monitoring developmental trajectories, and targeting treatment efforts. Researchers during the process examined effects of biological sex, age, and ASD diagnosis on resting-state encephalography (EEG) among a large, sex-balanced sample of youth with (N=142, 43% female) and without (N=138, 49% female) ASD collected across 4 research sites. Decreased EEG power across multiple frequencies was related to female sex and older age. Youth with ASD displayed reduced alpha power relative to peers without ASD, suggesting increased neural activation during rest. Associations between EEG and behavior varied by sex. Whereas power across various frequencies correlated with social skills, nonverbal IQ, and repetitive behavior for males with ASD, no such associations were observed for females with ASD. Research using EEG as a possible ASD biomarker considered individual differences among participants, as these features influence baseline EEG measures and moderate relations between EEG and critical behavioral outcomes. Failure to consider factors such as biological sex in such research risks defining biomarkers that misrepresented females with ASD, which hindered understanding of the neurobiology, development, and intervention response of this critical population.