Two babies for one pregnancy – also known as twins. There are two kinds, of course. Dizygotic twins come about when two eggs meet two spermatozoa, and produce two simultaneous babies in one womb; they are also known as fraternal twins. The monozygotic ones come about when the egg turned zygote, by some random quirk of cell division, splits into two embryos; the resulting two children share a vast majority of their genetic code, and look damn near identical, too.
Of course, having two people who share a great deal of their DNA can be useful in all kinds of scientific experiments, and particularly when it comes to genetic research itself. Scientific interest in twins has been around for centuries. One of the first people to look into twins systematically was 19th century English polymath Sir Francis Galton. He was greatly interested in individual differences between humans – everything from fingerprints to height, to behaviour and intellectual capabilities was a thing to be studied, measured, and compared. Galton was particularly keen on determining whether one’s abilities were hereditary. As he writes in the preface of his 1892 book Hereditary Genius, “each generation has enormous power over the natural gifts of those that follow, and [I] maintain that it is a duty we owe to humanity to investigate the range of that power, and to exercise it in a way that, without being unwise towards ourselves, shall be most advantageous to future inhabitants of the earth.” If that smacks of eugenics to you, it’s because Galton established the whole field, and coined the very term.
Questionable as his beliefs about humanity’s future may have been, Francis Galton nevertheless wanted to employ a good scientific method for testing his ideas on heritability while attempting to remove the inevitable factor that could muddy up any results – the influence of a common environment for two siblings growing up. As he explains in his 1875 paper History of Twins, by surveying the life histories of an identical pair of siblings, it is possible to find out why they would ‘grow unlike’ over time. Conversely, one could also find out why the characters of a dissimilar pair of twins “became assimilated under the influence of identical nurture.”
He was onto something, of course. These days twin studies comprise a whole genre of genetic research, and some of the most insightful are the ones of twins who have been brought up in separate homes, under the influence of vastly different environments. Sir Francis Galton was indeed one of the first behavioural geneticists – an interdisciplinary field that is still trying to untangle the debate on how much nature gives us, and how much can be attributed to nurture.
Bonus: Now go read this incredible story about two pairs of twins mixed up right after birth – it’s a twin study like no other.