Two minds, one body and a billion questions.

Conjoined twinning is one of the most fascinating and at the same time devastating human malformations. This is an extremely rare phenomenon. The occurrence is estimated to range from 1 in 50,000 births to 1 in 100,000 births [1], when identical twins are born physically connected to each other. They can be joined anywhere—head, chest, abdomen, hips, and so on [2]. In fact, there is a whole spectrum of cases with different degrees of bodily overlapping: from being joined by a thin sliver of skin to being extensively fused. The “fusion” can be so extensive that in some cases, it is no longer correct to talk about “twins” because there is only one individual with some extra organs [3].

Conjoined twins have been known to exist for centuries, yet there is very little understanding of this phenomenon. Common public questions are: How do conjoined twins live together? How do they eat, walk or manage any other daily routine activities? Do they share thoughts and can they read each other’s mind?

The answers to these questions are indeed different for different pairs of conjoined twins. For example, 27-year old Abigail “Abby” and Brittany Hensel are joined at the torso. They have two hearts, two spines, two sets of lungs, two arms and two stomachs. Below the waist, they are more like one body. Each twin controls her half of their body – Brittany, the left twin, can’t feel the right side of her body, and vice versa. Each twin manipulates one arm and one leg.

As infants, the initial learning of physical processes that required bodily coordination, such as clapping, crawling, and walking, required the cooperation of both twins, even standing up takes total coordination. Now as grown-ups they are incredibly well coordinated with this set-up, able to walk with a smooth gait, dribble a basketball, ride a bike, and even drive a car: both steer and Abigail controls the accelerator with her right foot. The really mesmerizing thing is watching them type on a computer, as both girls’ hands fly over the keys, but there is no verbal discussion of what they are writing [4].

For 98 percent of all sets of conjoined twins, each person has their own separate and distinct thoughts and feelings. But in the case of Tatjana and Krista Hogan [5], which occurs in only one in 2.5 million births, they share neural activity because their skulls are connected.

The girls are still too young to investigate their neurological wiring, but from the MRI scans, doctors have determined that there is a “thalamic bridge” that links one sister’s sensory input to the other, creating a conscious loop. Essentially, if one thinks a happy thought, the other can perceive it. When one sees an image through her eyes, the other receives the image milliseconds later.

With a few tens pairs of conjoined twins across the world today Abby and Brittany, Krista and Tatjana are defying the odds. And a fair answer to all the curious questions can be that they are able to do normal things, even though it takes a lot more effort for them than anyone can imagine.


— Mariia Filianina

  1. Mutchinick, O.M. Conjoined twins: a worldwide collaborative epidemiological study of the international clearinghouse for birth defects surveillance and research, Am J Med Genet C Semin Med Genet. 0, 274 (2011).
  2. Kaufman, M.H. The embryology of conjoined twins, Child’s Nervous System 20, 508 (2004).
  3. Savulescu, J. and Persson, I. Conjoined twins: philosophical problems and ethical challenges, Medicine and Philosophy 41, 41 (2016).
  4. Abby and Brittany: Joined for Life, BBC.


  5. Ryan, D. Through her sister’s eyes: conjoined twins Tatiana and Krista were extraordinary from the beginning. The Vancouver Sun[On-line] (2012). Available: