In Washington state in 2002, Lydia Fairchild nearly lost custody of her three children, when a test revealed that none of them shared her DNA. It turned out that Fairchild’s body was populated with cells from a non-identical twin she’d unknowingly had before birth, making her, in effect, the biological aunt of her own children.
The technical term for Fairchild is a ‘human chimera’: a human being composed of cells that are genetically distinct. The phenomenon can happen artificially, through a transfusion or transplant, or naturally, as in Fairchild’s case, through the early absorption of a twin zygote. Only 100 cases of natural chimerism are documented, but there may be many more. Scientists estimate that 36 per cent of twin pregnancies involve a vanishing twin. Most such twins likely disappear without a trace, but some get partly absorbed into their neighbour. The survivor is unlikely to learn of their lost sibling’s genetic presence, unless an unrelated test or procedure inadvertently reveals it. Go in for a routine cheek swab, come out with a twin.
Many find the idea of unknowingly carrying the vestiges of their twin unsettling. One person I told about Fairchild instantly burst into tears. I’m less perturbed by it, likely because I’ve known I have a twin for decades. My own twin Julia survived our joint gestation (rather than me, what, eating her? Gross!) If I find out I’ve got another one in there somewhere, it won’t be my first rodeo.
What mainly interests me about human chimeras are the philosophical, not the personal, implications. What should we say, metaphysically, about Fairchild and her ilk?
Journalists reporting on Fairchild’s case didn’t quite know what to make of it. ‘She’s her own twin,’ proclaimed ABC News. ‘The many yous in you,’ intoned Ed Yong in National Geographic. ‘A Guide to Becoming Two People at Once,’ wrote Maia Mulko in Interesting Engineering in 2021. Such headlines are clickbait because they challenge a standard presumption of modern Western culture, so basic as to go unstated. Westerners generally think that each person is physically discrete, cleanly distinguished from all other people by their location, solo, within an unbroken continuum of skin.
Actually, though, human chimeras leave this assumption intact. Fairchild isn’t two people in one, because the mere presence of human DNA doesn’t indicate the presence of a person. Any stray hair you leave on your pillow overnight is biologically human, but that doesn’t mean that, every time you shed hair, you’re multiplying the number of people in the room. Personhood requires something more than a particular type of genetic material: it arises only with the larger-scale structural organisation of that material, which permits capacities like consciousness, thought and moral agency. At the macro level that matters for personhood, Fairchild is a singleton.
Still, the one-person-per-body assumption is worth questioning, and there’s a much more convincing example of its violation at hand. Conjoined twins, unlike chimeras, contain only one genetic cell line. But (when two heads are present) they overwhelmingly consider themselves to be two unique, distinct beings, despite sharing a body. It’s typical for them to speak of themselves as individuals, and to develop a personality and tastes different from each other’s. Their families and friends, too, think of them as two people who just happen to be physically attached.
The case of conjoined twins reveals the falsity of the assumption that bodies correlate one-to-one with people. Recognising this has large implications. If one body can contain two people, why couldn’t one person be spread across two bodies? Why couldn’t that person be me, or you?
Singletons are always implying that twins aren’t fully distinct people, but rather a single person, split or duplicated. Antonio asks of Sebastian and Viola in Twelfth Night: ‘How have you made division of yourself? An apple, cleft in two, is not more twin than these two creatures.’ The Nuer people of South Sudan don’t hold a ceremony when one twin dies, because they believe the deceased lives on in their surviving twin. And any pair of twins you know will have tales of being given a single birthday present to share, or being referred to as ‘the twins’ instead of by their individual names, and being treated as essentially interchangeable by teachers, friends or relations.
For much of my life, I’ve vigorously resisted this attitude. Sure, there are various ways in which one twin can be a stand-in, stunt double, accessory or control for the other. Julia and I never switched classrooms or sexual partners (a twin rumour that’s mainly fake news), but we once startled a customer of the bookstore chain we both worked at, when I sent him to her branch after he’d called in at mine, and I appeared to be waiting for him when he arrived at the other store. As kids and teens, Julia and I were pros at pooling resources, whether of the mental or material kind. We collaborated on creative projects, studied for exams together, and each saw our wardrobe magically expand when the other bought clothes. I outsourced many life experiments to Julia, my bolder counterpart: she tried out driving, sex and spinal surgery first, and her dalliance with peroxide helpfully took blondeness off the table for us both for the rest of our lives.
But actual metaphysical merger? No way, I used to think. Julia and I have distinct personalities: she’s the assertive extrovert, Susan in The Parent Trap (1961); I’m Sharon, the amenable introvert, chiefly enthused about books and my cat. We now live independent lives in different countries, 19 airplane hours from each other. I don’t have access to Julia’s calendar, let alone her thoughts; when someone steps on her foot, I don’t feel it. If there’s any basis for thinking we’re one person, I’ve always assumed, it must be some incoherent or mystical conception of personhood that it’d be not only unprofitable but uncharitable to examine.
Still, I’ve been thinking more about twins recently, and I’m no longer so sure about that. There now seem to me at least three ways in which twins can genuinely function as a single person.
I trust her memories of our distant past as much, if not more, than my own
First, twins can share a mind. I’m not referring to telepathy here, which is a dubious matter of extra-sensory communication between minds. Instead, I’m referring to twins using each other’s minds – or, maybe better, using their own mind but outside the skull we normally associate with them.
In their paper ‘The Extended Mind’ (1998), the philosophers Andy Clark and David Chalmers argued that, to identify something as an instance of thought, we simply need to identify a process that plays the functional role that thinking does. It doesn’t matter where the process is. For instance, if your use of your phone’s calculator plays the same role for you as your tallying up the numbers internally does, we should see both acts as forms of thinking and, provided that your phone is deeply and reliably enmeshed in your life, it and your brain should be classed as a single cognitive system.
If your mind can extend to an inanimate object, why not also an animate person? Some empirical work in social psychology supports the idea. Daniel Wegner’s studies of what he terms transactive memory explore how couples or groups use each other as repositories of distinct forms of information, allowing each to recall more than they would singly. Couples also ‘cross-cue’ each other, remembering in tandem by throwing prompts back and forth till they trigger each other’s recollections – ‘in a sense,’ as Clive Thompson suggested in Slate, ‘Googling each other.’
Any couple could think jointly this way, but close twins are surely among the world’s best instances. Julia and I did practically everything together till I left the country at 21: we attended the same schools, were interested in the same subjects, lived with our parents through college, had many of the same friends, and took all our vacations together. My memory for detail is embarrassingly bad, so it’s handy to have Julia at hand to recall all this for me. I trust her memories of our distant past as much, if not more, than my own, and when I’m dredging up the more recalcitrant secrets of my personal history, it doesn’t feel all that different from asking her to do it instead.
A second way in which twins can share personhood is by acting as a plural agent. Philosophers have spelled out the concept of plural agency in different ways, but according to Bennett Helm’s account, what’s crucial is that two or more people have genuinely joint concerns and values. They recognise a set of common aims, commit to acting as a group to pursue them, and care about the group itself, as an aspect of their own agency. In this way, they create and act from a new, unified entity alongside their own individual selves.
Twins are a compelling example of a plural agent, if anyone is. As Laura Spinney wrote on twins in Aeon, ‘in the best instances’ they possess ‘absolute mutual trust, a highly developed theory of the other’s mind, and an ability to work together that surpasses that of any other human dyad.’ Julia and I were like this as kids, in a way that probably made our singleton friends envious. I could count on my twin to enthusiastically sign up for any plan I proposed, whether it was co-creating a novel (I wrote; Julia illustrated), throwing a party (‘sea-themed – in a lighthouse!!’), or making someone cool like us (who can resist the seductive power of twins?) We executed our various missions jointly, with almost no friction. It was like having an extra jetpack strapped to your will.
How can I reconcile my sense that my self is both separate from Julia’s and shared with her?
Finally, twins can share not only cognition and action, but also an identity. People who regularly form a plural agent in important and extensive areas of their lives come to deeply identify with one another, and their relationship becomes central to who they each individually are. This is likely what Aristotle had in mind when he referred to a close friend as ‘another self’, and it explains why the death of an intimate can cause such deep mourning. In losing a dear friend, you’ve lost the plural person you formed together. If you acted as that person in wide and deep domains of your life, it’s not purely metaphorical to say that part of your own self has been ripped from your chest.
Not all twins get along but, when they do, the bond they share is special. When one twin dies, the surviving twin’s score on the Grief Experience Inventory is, on average, the highest on the planet. I read once about a conference held for twins that included a session on grieving a lost twin. Apparently, not one of the many conference attendees turned up to that one: they couldn’t stand it. I told Julia about this, and she just nodded. We wouldn’t go either.
I now think that these three phenomena – the sharing of cognition, agency and identity – support the idea that (close) twins share personhood to a significant degree. But I still resist the suggestion that Julia and I are simply the very same person. That would imply some pretty wild things: for instance, that if Julia committed a crime, there’d be no moral difference between punishing her for it and punishing me; that I’m her kid’s mother, rather than her kid’s aunt; and that whoever I’m dating, she’s dating, too. Pure chaos!
How can I reconcile my sense that my self is both separate from Julia’s and shared with her? Lately I’ve been thinking that the problem comes from seeing personhood as unitary and static. What if it’s dynamic and discontinuous instead? What if a person isn’t only something you are, but also something you do? Since what you do varies over time, you could then move in and out of shared personhood with another person, at different times and in different domains of life, and to different degrees, depending on how you’re interacting with them.
My life bears out this picture of twins dipping in and out of shared personhood over time. Julia and I haven’t lived in the same country for more than two decades, and the occasions when the border between us seems to blur are rarer now than when we led our daily lives alongside each other and in concert. But those experiences of merger still arise, usually when we spend an extended amount of time together on vacation. In one striking recent instance, when Julia and I were both pressed for time, I found myself absent-mindedly offering to go to the restroom to pee on her behalf. Do I really think I share a bladder with my twin? No. Do I think I share personhood with her? These days, I’m giving a qualified yes.
When people in Western culture imply that twins are one person, what they often seem to mean is that twins are less than one person: that neither I nor Julia, for example, achieves full personhood by virtue of our overly close enmeshment with each other. ‘It’s high time you quit being twins and began being people,’ says one sister’s boyfriend in the teen romance novel Double Trouble (1964). ‘Separate people.’ As if those two things were equivalent.
Twins understandably react poorly to the suggestion that they’re less than full people, since a powerful set of norms tells us that only full people can be moral agents, rights-bearing citizens, and beings of dignity and worth. Being half a person, we assume, is like being no person at all.
But what if the underlying premise that full personhood requires closely guarded separateness from others is wrong? It’s only relatively recently in our species that the best life has been portrayed as one of self-governed individual action, free of the influence and demands of others. For most of the human past, across most of the planet, personhood has been grounded in social relationships. Who you are has been seen as a function of how you fit into an interdependent network of kin and communal relations. Twins who share personhood can be seen as a problematic throwback to this benighted past. (‘Boundaries!’ we all scream.) Or such twins can be seen as a vivid reminder of the truth and beauty of the older picture.
We don’t really need chimeras or twins to reveal the deeply relational nature of our species. The experience of merged personhood is common in many other types of close couples. New parents speak of the startling sensation of having a part of themselves exist outside their body: their infant, a piece of their actual heart, sleeping quietly in the next room. Frank Sinatra croons to his lover: ‘I’ve got you under my skin … so deep in my heart that you’re really a part of me.’ Michel de Montaigne wrote, after his best friend’s death, that he’d become ‘so formed and accustomed to being a second self everywhere that only half of me seems to be alive now.’ We can take all of this figuratively – as a poetic expression of strong feeling – or we can treat it as a literal and defensible metaphysical stance. After all, once we twins have embraced breaking the body barrier, what’s stopping singletons from doing it, too? What makes you so sure that all of you is contained within that single envelope of skin?