Are differences between twins a result of mutual rivalry?

Area of Science:

• Psychology
• Twin Studies
• Gender Studies

Background:

• The Szondi test's sexual index measures
• Internal sex, or gender, is distinct from external sex.
• Twin studies allow for the examination of genetic and environmental influences on psychological traits.

Purpose of the Study:

• To investigate the congruence of internal sex (gender) in same-sex twin pairs using the Szondi test's sexual index.
• To explore the role of competition in observed incongruence of gender within twin pairs.
• To examine gender differences in the congruence of internal sex.

Main Methods:

• Comparison of sexual index scores in 62 monozygotic (MZ) and 23 dizygotic (DZ) twin pairs.
• Analysis of incongruence in sexual index scores within twin pairs.
• Hypothesis testing regarding competition for dominance and gender differences.

Main Results:

• Approximately one-third of MZ twin pairs showed incongruence in their sexual index scores.
• Competition for dominance was identified as a key factor underlying incongruence, particularly in male pairs.
• Gender congruence was more pronounced in female twin pairs compared to male pairs.

Conclusions:

• Internal sex (gender) and external sex should be considered as parallel variables.
• Competition for dominance, linked to masculinity, contributes to gender incongruence in males.
• Divergence in twin test results from the general population can be attributed to competitive dynamics within pairs.