CSO Frontier Series outputs may use new methods which are under development and/or data sources which may be incomplete, for example new administrative data sources. Particular care must be taken when interpreting the statistics in this release.
Learn more about CSO Frontier Series outputs.
GUI is the national, longitudinal study of children and young people in Ireland. It is a collaborative study between the Central Statistics Office (CSO) and the Department of Children, Disability and Equality (DCDE).
The GUI study was established in 2006 and is a unique longitudinal study that originally followed two groups of children in Ireland: Cohort '98, who joined the study when they were 9-years old, and Cohort '08, who joined the study when they were 9-months old. These two cohorts have been surveyed at regular intervals since then. Cohort '98 were aged 25 at their last wave of data collection and the main results of the survey were published in January 2025. Cohort '08 will be aged 17/18 in the next wave of data collection, planned to begin in Q3 2025. In September 2024, the CSO and DCDE launched a third GUI cohort: Cohort '24. Households with 9-month-old babies are invited to participate up until Q3 2025.
Between September 2008 and April 2009, more than 11,000 families with 9-month-old children were interviewed as part of Cohort '08, with follow up waves being completed when the children were aged 3 years, 5 years, 7/8 years, 9 years, and 13 years. This analysis uses data from when the children were 9-months old, collected from families between September 2008 and April 2009 (Wave 1). Of the 11,405 children whose families were interviewed at Wave 1; 10,736 (94.1%) were singleton children and 669 (5.9%) were non-singleton (twins or triplets). This Frontier Series Research Paper is the first release utilising data from non-sample twins in the GUI survey (see Background Notes for further details).
There are a small number of triplets in the GUI study, and for the purposes of this release the terms non-singleton or twin are used for all multiple births.
Throughout this release, the term singleton refers to a child who does not have a twin or triplet sibling and the term non-singleton (or twin) is used for a child who has a twin or triplet sibling.
The primary caregiver questionnaire included items related to pregnancy and birth, which were administered only when the primary caregiver was identified as the birth mother. As the majority of these questionnaires were completed by the mother of the baby, this report will refer to the respondent as "the mother" rather than "the primary caregiver" for clarity and consistency.
Cohort '08 is made up of the families of more than 11,000 children who were born between 1 December 2007 and the end of June 2008. The first wave of data collection for this group took place between September 2008 and April 2009.
The parents of more than 650 twins or triplets were interviewed in the first phase of GUI Cohort '08 when the children were 9-months old and data was collected on each 9-month-old. In some households, both twins were sampled for interview independently so both twins’ data was published in the subsequent, weighted data files. In other cases, only one twin was sampled for interview, in these cases the data for the other twin was not part of the data analysis. The data in this release includes the previously unused data on non-sampled twins.
The figures presented are therefore unweighted as some of the twins are not part of the original weighted sample, as such, the results presented should be interpreted with caution and are not nationally representative. In addition, due to the unequal sample size between the twins and singletons, comparisons are illustrative rather than empirical.
The CSO would like to thank the many contributors to Growing Up in Ireland Cohort '08. In particular, we would like to thank the parents who filled out the additional twin questionnaires for this survey, for their responses and for recognising the importance of facilitating the collection of this highly significant data.
As well as the strict legal protections set out in the Statistics Act, 1993, and other existing regulations, the CSO is committed to protecting individual privacy. All identifiable information is removed so that no individual or household can be identified in the published data.
Learn about our data and confidentiality safeguards, and the steps we take to produce statistics that can be trusted by all.
Introduction
The Central Statistics Office (CSO) has today (26 September 2025) published a Frontier Series research paper on the twins who were part of the Growing Up in Ireland (GUI) Cohort '08 at 9-months old. This is the first of two research papers that will be published on twins in the GUI Cohort '08 survey, with the second focusing on their lives at 13 years of age.
Cohort '08 households with children born in 2008 first joined the survey when the children were 9-months old. The most recent data collection occurred when the children were 13 years old, and they will be revisited again over 2025-2026 when the children will be aged 17/18 years old. Since the beginning of GUI Cohort '08 data collection, responses were collected from twins in households as well as individual children, or singleton children. Utilising all of this data for the first time, this release shares analysis of the experiences of households with twins compared with single children.
This release is part of a Frontier series which presents a developmental overview of twins at two distinct life stages: today’s release at infancy and our next release will cover early adolescence. By examining 9-month-old and 13-year-old twins, we gain valuable insight into how twin dynamics evolve over time and influence physical, cognitive, emotional, and social growth.
By analysing twins at these two pivotal stages, this Frontier series aims to provide a holistic understanding of twin development across time. It highlights the continuity and change in their relationship, the impact of shared experiences, and the importance of fostering both connection and individuality.
However, due to the small number of twins involved, and that this is a CSO Frontier Series research paper, particular care must be taken when interpreting these results. The figures presented are unweighted as some of the twins are not part of the original weighted sample, as such, the results presented should be interpreted with caution and are not nationally representative. In addition, due to the unequal sample size between twins and singletons comparisons are illustrative rather than empirical (see Editor’s note below and Background Notes for further details).