Personhood and Conjoined Twins

Conjoined twins are frequently brought up by the pro-life community as a kind of “gotcha” response to bodily autonomy arguments for abortion rights. Typically, such an argument goes like this:

Conjoined twins are two people who share the same body. Each has bodily autonomy – yet neither twin can violate the other’s right to life by killing the other, despite the fact each uses the other’s body. Thus, bodily autonomy does not permit the killing of someone who uses another’s body for survival.

The defender of bodily autonomy usually has a straightforward reply, focusing on the fact that the woman has prior claim on her body. Unlike the conjoined twins, the woman had existed long before the unwanted fetus. This relevant dissimilarity makes the idea the woman and the fetus must necessarily have “joint ownership” over the woman’s body much less compelling.

(One might point out that most would agree that while two equally ancient Indigenous groups might be obligated to share their land equitably, this does not imply that the Indigenous are likewise obligated to share their land with an invading colonial force; the Indigenous have a prior claim to the land that the colonial force does not, and that matters morally.)

But I think the case of conjoined twins has interesting implications for the pro-life side. In granting that the conjoined twins represent two people sharing the same body, the pro-life side seems to be conceding that being a human organism is not equivalent to being a person. What matters to personhood, even to the pro-life side, is having an independent mind, even if that mind is sustained by the same body that also sustains another mind.

To understand this, let’s take a look at the basic biology behind conjoined twins, and a few relevant examples.

Biology of Conjoined Twins

A plausible explanation of the origins of conjoined twins goes like this. A sperm cell fertilizes a single egg cell. The resulting embryo begins to split into two – but only does so partially. The resulting fetus develops two sets of some organs but shares some others.

It is difficult to understand where in this process there could be said to be “two human organisms”. Both twins are completely dependent on the unified body to carry out biological functions. Neither twin can (typically) sustain itself without the other. Yet it seems equally obvious that the conjoined twins represent two people – except in the exceedingly rare cases we will examine shortly. Conjoined twins may share different political views; they seemingly ought to have two rights to vote, not one; they have two sets of experiences.

Most people, both pro-life and pro-choice, would agree that Abby and Brittany are equally persons and that it would be highly immoral to conduct a surgery that would kill one while keeping the other alive (even if the other were to have a better quality of life.) But there are rare cases where this intuition does not hold water. Those cases include ones where an embryo partially divides, but only one brain develops in the resulting human organism. Consider the case of Lakshmi Tatma, a baby born of a conjoined twin in which one twin never developed a brain.

In this case, it seems that there is one human organism, and it seems that there is only one person – the other conjoined twin, whether one regards it as a separate human organism or not, is obviously not a person. There is no moral difficulty to removing the other conjoined twin via surgery; indeed, Lakshmi Tatama had the surgery and recovered well.

Thus, we see the defenders of the “human organism is equivalent to human person” view are in a double bind. If they say that conjoined twins are two human organisms and two persons, then it seems Lakshmi’s surgery was immoral, because it killed a human organism (or person) in order to further Laksmi’s quality of life. But Lakshmi’s surgery was not immoral, and so the conjoined twins are either not two human organisms or not two persons. If they say that conjoined twins are one human organism, then it seems that Abby and Brittany Hensel are one person. But Abby and Brittany are not one (singular) person. Under either approach, “being a human organism” is not reducible to “being a person”, or vice versa.

By contrast, the pro-choice view provides an easy explanation for the dilemma. Abby and Brittany have two minds (with mind here loosely referring to a consciousness-sustaining brain), so they are two people. Lakshmi’s twin lacks a mind, and so Lakshmi’s twin is not a person; Lakshmi, by contrast, has a mind and therefore is a person.