We used a mixed-methods, sequential, exploratory design. In the qualitative phase, an in-depth interview approach was used to identify the properties and dimensions to be included in the Opposite-Sex Relationship Questionnaire for Male Adolescents (OSRQMA). In the quantitative phase, the psychometric properties of the OSRQMA were evaluated according to the concepts of face, content, and construct validity. Reliability and stability were assessed with Cronbach's alpha (α) and McDonald's coefficient omega (ω), and with test–retest analysis respectively. A preliminary questionnaire was developed, including 61 items that emerged from the qualitative phase of the study. Based on the impact scores for face validity, and the cutoff points for the content validity ratio (CVR) and content validity index (CVI), the preliminary questionnaire was reduced to 53 items. The Kaiser criteria (eigenvalues >1) and scree plot tests demonstrated that 22 items forming four factors, which were labelled ‘innate predilection’, ‘abstinence’, ‘family attitudes’, and ‘peer pressure’, were optimum, accounting for an estimated 53.449% of the variance. These scales had acceptable levels of internal consistency (α = .854, ω = 0.977) and stability (r = .892, p < .001).

Sex differences in religion are well known, with females generally being more religious than males, and shared environmental factors have been suggested to have a large influence on religiousness. Twins from opposite-sex (OS) and same-sex (SS) pairs may differ because of a dissimilar psycho-social rearing environment and/or because of different exposures to hormones in utero. We hypothesized that OS females may display more masculine patterns of religiousness and, vice versa, that OS males may display more feminine patterns. We used a web-based survey conducted in Denmark, which is a secular society. The survey included 2,997 twins aged 20–40 years, identified through the population-based Danish Twin Registry. We applied la Cour and Hvidt's adaptation of Fishman's three conceptual dimensions of meaning: Cognition, Practice, and Importance, and we used Pargament's measure of religious coping (RCOPE) for the assessment of positive and negative religious coping patterns. Differences between OS and SS twins were investigated using logistic regression for each sex. The analyses were adjusted for dependence within twin pairs. No significant differences in religiousness and religious coping were found for OS and SS twins except that more OS than SS females were members of the Danish National Evangelical Lutheran Church and fewer OS than SS females were Catholic, Muslim, or belonged to other religious denominations. Moreover, OS males at age 12 had higher rates of church attendance than did SS males. This study did not provide evidence for masculinization of female twins with male co-twins with regard to religiousness. Nor did it show any significant differences between OS and SS males except from higher rates of church attendance in childhood among males with female co-twins.