Funding for this research was provided by:
Economic and Social Research Council (1948633)
Received: 1 October 2021
Accepted: 6 December 2021
First Online: 23 December 2021
: 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 were in compliance with the standards specified by this body.
: Not applicable.
: The authors declare that they have no competing interests.