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 by DCDE 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, which began in Q4 2025. In September 2024, the CSO and DCDE launched a third GUI cohort: Cohort ’24. Households with 9-month-old babies were 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 13 years old, collected from families between July 2021 and June 2022 (Wave 6 at 13 years). Of the 6,827 children whose families were interviewed at Wave 6; 6,420 (94%) were singleton children and 407 (6%) were non-singleton (twins or triplets). This Frontier Series Research Paper uses data from sampled and non-sampled twins in the GUI survey (See Background Notes for further details).
Although the GUI study includes a small number of triplets, for the purposes of this research paper, the terms non-singleton or twin are used to refer to all multiple births. Throughout the release, singleton describes a child without a twin or triplet sibling, while non-singleton (or twin) refers to a child who has one.
In Wave 6 of the GUI study, questionnaires were completed by the young person, their Primary Caregiver (typically the mother), their Secondary Caregiver (usually the father), and the principal of the school they attended.
Cohort ’08 comprises families of more than 11,000 children born between 01 December 2007 and the end of June 2008. The first wave of data collection took place from September 2008 to April 2009, with the sixth wave occurring between July 2021 and June 2022, when the children were 13 years old.
During Wave 6, parents of more than 400 twins or triplets participated, and data was collected for each 13-year-old child. In some households, both twins were independently sampled and included in the weighted data files. In others, only one twin was sampled, meaning the second twin’s data was not originally part of the analysis. This release now incorporates previously unused data from those non-sampled twins (See Background Notes for further details).
As a result, the figures presented are unweighted, since some twins fall outside the original weighted sample. These findings should therefore be interpreted with caution and are not nationally representative. Additionally, due to the unequal sample sizes between twins and singletons, any comparisons made are illustrative rather than statistically definitive.
The CSO extends its sincere thanks to all contributors to the Growing Up in Ireland Cohort ’08 study. We are especially grateful to the parents who completed the additional twin questionnaires, whose participation has been vital in enabling the collection of this important data.
In line with the legal safeguards outlined in the Statistics Act, 1993, and other regulations, the CSO remains committed to protecting individual privacy. All identifying information has been removed to ensure 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 (03 November 2025) published the second in a two-part series on the twins who took part in the Growing Up in Ireland (GUI) Cohort ’08 survey. This latest research paper explores the lives of twins who were part of the GUI Cohort ’08 at 13 years old following our earlier release, Growing Up Together: Twins in the Growing Up in Ireland ’08 Study at 9-Months Old.
Since data collection first began for GUI Cohort ’08, responses were collected from twins in households as well as individual children, or singleton children. Utilising all of this data, this release shares analysis of the experiences of households with twins compared with singleton children.
By studying twins at 9-months and 13 years of age, we gain meaningful insight into how twin relationships develop over time and shape physical health and development, emotional well-being, and social growth. These two distinct life stages offer a valuable lens through which this research paper, published as a CSO Frontier Series, explores the evolving nature of twin dynamics.
The research aims to present an overview of twin development, emphasising both the continuity and transformation in their interactions, the influence of shared experiences, and the balance between connection and individuality.
Given the limited number of twins included and the nature of this CSO Frontier Series Research Paper, findings should be interpreted with care. The data presented is unweighted, as some twins fall outside the original weighted sample, making the results non-representative at a national level. Additionally, comparisons between twins and singletons are illustrative rather than statistically robust due to unequal sample sizes (See Editor’s Note and Background Notes for further details).