Received: 13 December 2021
Revised: 1 June 2022
Accepted: 7 June 2022
First Online: 24 June 2022
: This work was supported by the Economic and Social Research Council Advanced Quantitative Methods Studentship (Project reference: 1948633) which was awarded to AK. The funders had no role in any element of this research. The remaining authors declare no competing interests.
: Ethical approval was originally granted for the study in 2007 by the University College London (UCL) Committee for the Ethics of non-National Health Service Human Research. In 2018, ethical approval for the continuation of the study was again granted by the UCL Research Ethics Committee (Project ID 1624/004). Written informed consent was provided by all Gemini families. All aspects of data collection and storage complied with the standards specified by this body.