Zygosity questionnaires have been developed to determine the zygosity of a pair of twins based on their overall similarity (“as alike as two peas in a pod”) and how often they are confused by other people.

TRA has a Peas-in-a-Pod questionnaire with three questions:

• Peas-in-a-Pod questionnaire for twin children
• Peas-in-a-Pod questionnaire for twin adults

The questionnaire is based on research by S Ooki and can accurately determine zygosity for 93% of twin pairs:

• Zygosity Diagnosis of Twins by Questionnaire
• Zygosity Diagnosis in Young Twins by Questionnaire for Twins’ Mothers and Twins’ Self-reports

TRA have separately validated this questionnaire:

• Why Accurate Knowledge of Zygosity is Important to Twins