Table 2 -.
Studies on perceptual and cognitive traits in opposite-sex and same-sex twins
| Trait | Publication | Setting | Numbers of opposite-sex (OS) and same-sex (SS) twins | Is there a comparison between OS and SS dizygotic (DZ) twins only?a | Age | Assessment | Findings OS vs. SS females | Findings OS vs. SS males | Suggests evidence for masculiniza-tion of OS femalesb | Suggests evidence for demasculiniza-tion of OS malesb |
|---|---|---|---|---|---|---|---|---|---|---|
| Academic performance | Ahrenfeldt et al., 2015a | Denmark | 968 OSF 2198 SSF 966 OSM 2194 SSM (breakdown not provided) |
Yes | 15–16 years | Ninth grade test scores obtained from registers | OSF obtained slightly lower test scores in mathematics than SSF – opposite to what was expected | No significant differences | − | − |
| Auditory system functioning | McFadden, 1993 | Texas, USA | 17 OSF 32 SSF (DZ) 46 SSF (MZ) 17 OSM 24 SSM (DZ) 34 SSM (MZ) |
Yes | OS: mean = 20.8 years SS (DZ): mean = 18.7 years SS (MZ): mean = 21.3 years |
Spontaneous otoacoustic emissions (SOAEs) | SOAE frequency lower for OSF than for SSF | No significant differences | + | − |
| McFadden et al., 1996 | Texas, USA | 18 OSF 32 SSF (DZ) 42 SSF (MZ) 17 OSM 24 SSM (DZ) 34 SSF (MZ) |
Yes | OS: mean = 20.7 years SS (DZ): mean = 18.2 years SS (MZ): mean = 21.3 years |
Click-evoked otoacoustic emissions (CEOAEs) | No significant differences, but spectral power of CEOAEs marginally lower (more masculinized) for OSF than for SSF | No significant differences | + | − | |
| Expressive vocabulary | Galsworthy et al., 2000 | England and Wales | 974 OSF 994 SSF (DZ) 1126 SSF (MZ) 974 OSM 992 SSM (DZ) 952 SSM (MZ) |
Yes | 2 years | Parent report: MacArthur Communica-tive Developmental Inventory (MCDI) - Short form | OSF lower MCDI scores (more masculinized) than SSF | OSM higher MCDI scores than SSM | + | + |
| Van Hulle et al., 2004 | Wisconsin, USA | 112 OSF 61 SSF 8DZ) 64 SSF (MZ) 112 OSM 64 SSM (DZ) 52 SSF (MZ) |
Yes | 20–38 months | Parent report: MCDI - Short form | OSF lower MCDI scores than SSF | OSM higher MCDI scores than SSM | + | + | |
| Visuospatial cognition | Vuoksimaa et al., 2010b | Finland | 120 OSF 351 SSF 110 OSM 223 SSM |
Yes | Mean = 22.4 years | Mental Rotation Test (MRT) | OSF performed better than SSF | No significant differences | + | − |
| Heil et al., 2011 | Germany | 100 OSF 100 SSF (DZ) |
Yes | 19–39 years, mean = 23.6 years | Mental rotation test from an MRT questionnaire | OSF performed better than SSF | NA | + | NA |
NA: Not available, OS: opposite-sex, SS = same-sex, F = females, M = males, MZ = monozygotic, DZ = dizygotic, UZ = unknown zygosity
Yes: The study makes a separate comparison of OS vs SSDZ twins, although it includes both MZ and DZ twins.
No: The study does not make a separate comparison of OS vs SSDZ twins, thus, including MZ twins in all analyses.
DZ twins only: The study includes DZ twins only. Thus, no MZ twins are included in the study.
+: The study provides evidence for masculinization of OS females/demasculinization of OS males.
−: The study provides no evidence for masculinization of OS females/demasculinization of OS males.
+/−: The study provides evidence for masculinization of OS females/demasculinization of OS males in some cases (e.g. in some investigated measures, in some age groups or in some statistical analyses), but not in all.