The IVF design is a third natural experimental design that has been made possible by medical advances over the last several decades. Its design is consistent with prenatal cross-fostering approaches used in the life sciences. Assisted reproductive technologies such as IVF are an increasingly common means of conception, and it is estimated that more than 5 million children have been born via IVF since 1978 when the first IVF baby was born (https://www.eshre.eu/Guidelines-and-Legal/ART-fact-sheet.aspx). This phenomenon has created a new opportunity to examine whether associations between parenting and child behavior might vary as a function of whether the child is genetically related to both, one, or neither parent.
