Twins and Look-Alikes: Perspectives on Human Reproductive Cloning
The 1996 birth of Dolly, the cloned Scottish lamb, captured worldwide attention when the news was announced in 1997. Scientists and the public were divided as to the medical, behavioral, and moral implications of possible human reproductive cloning (HRC), although the reactions were mostly negative. Shortly thereafter, I was contacted by my colleague, Law Professor Owen Jones, then editor of Jurimetrics, a journal dedicated to issues surrounding law and behavior — he requested a paper addressing the behavioral implications of HRC from a twin-based perspective. I agreed to do this because I believed there was a great deal of misinformation and misunderstanding that needed correction and explanation.
Virtually all the published responses I had read since Dolly’s birth was announced had failed to reference the psychological situations of monozygotic (MZ) twins, the best human models for rational debate and discussion of this topic. It occurred to me that other human models, such as look-alike parent-child pairs and look-alike sibling pairs, would also be informative. Composing the paper for Jurimetrics also made me consider the uniquely defining features of twinship. I saw this an important task because members of the scientific community and members of the media often referred to clones as ‘twins’ or as ‘delayed twins’, labels I felt were inappropriate. I also suggested referencing possible human clones as intergenerational clones (IGC) to preserve the fact that they would be born years apart, unlike twins who are born close in time.
The definition of a clone and the features that distinguish twinship from clones were described in Segal (Reference Segal1997). First, it is important to understand that MZ twins are clones, but clones are not identical twins. MZ twins are clones because they are ‘precise copies of a molecule, cell, or individual plant or animal’. In contrast, clones are not twins, as I explain later below. First, I will present challenges to some of the negative responses to HRC; however, these challenges do not mean that HRC should proceed if available. That is because of issues surrounding cost and safety, and the likelihood of unanticipated concerns.
Challenges to HRC
A number of challenges were indicated in a report issued by the National Bioethics Advisory Board in response to Dolly’s birth (National Bioethics Advisory Commission [NBAC], 1997). Unfortunately, the authors of that report failed to cite twin research findings that could shape the scientific debate in a responsible and meaningful way. For example, it was asserted that ‘Common experience demonstrates how distinctly different [identical] twins are, both in personality and in personhood.’ It was also argued that others cannot help but ‘imbue identical bodies with some expectation that identical persons occupy those bodies since body and personality remain intertwined in human intuition’. When I read those passages, it seemed to me that if MZ twins are truly so different, then there is no cause for concern. However, research evidence shows that identical twins are more alike in personality, on average, than any other pair of people, but it is not perfect similarity — the MZ intraclass correlations are about .50 (Tellegen et al., Reference Tellegen, Lykken, Bouchard, Wilcox, Segal and Rich1988). In fact, people often exaggerate the small behavioral differences between MZ twins whom they know.
Other challenges that have been raised over the years are listed below, followed by my comments:
Cloned children would experience social stigma. MZ twins are highly visible and recognizable in the population and typically do not face ridicule due to their matched appearance. If a parent took their cloned child to a playground, it is highly unlikely that anyone would suspect their true biological relationship. Going unnoticed means that stigmatization would not occur.
Cloned children would undergo diminished autonomy. Many interviews reveal that MZ twins feel both part of a pair, as well as an independent individual. I have studied MZ twins reared apart from birth, some of whom were apprehensive about meeting their co-twin, due to fear of reduced autonomy. However, having met, none of them felt as though they had lost a part of themselves.
Revised family relationships would harm family members. Most parents are excited to observe their own traits in a child. However, how would they feel observing their partner’s traits in a son or daughter? I found an answer to this question when I interviewed Andrew and Elad Dvash-Banks, a same-sex couple who had twin boys created via egg donation and in vitro fertilization (IVF); see Segal (Reference Segal2023). Each parent had provided sperm leading to the conception of one twin, replaying the reproductive process of superfecundation. It turned out that both fathers delighted in seeing their partner’s traits in the twin son they had not conceived and described a special relationship with that child. This makes sense because the traits that drew the partners together would also be celebrated in their children. I have labeled this concept assortative cross-parenting (Segal, Reference Segal2024a).
Parental expectations for cloned children would be increased. Most mothers and fathers have educational and occupational expectations for their children, and donor parents would not be an exception. However, donor parents are likely to have special insights into their child’s talents and interests, laying the basis for understanding and acceptance.
Clones Are Not Twins
Over the years, I have developed and refined the criteria distinguishing twins and clones in a series of articles (Segal, Reference Segal1997, Reference Segal2002a, Reference Segal2024a, Reference Segal2025a), presentations at conferences (Segal, Reference Segal2002b, Reference Segal2024b), and a talk at TedX — Manhattan Beach (Segal, Reference Segal2025b). Occasional variations to the criteria listed below are described in the original sources; for example, twins are sometimes delivered days apart, but they all remain valid.
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Twins are conceived at the same time, whereas donors and clones are conceived at different times.
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Twins share a womb, but donors and clones have separate wombs.
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Twins are delivered on the same day, whereas donors and clones are delivered in different years.
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Twins are members of the same generation, whereas doners and clones are members of different generations.
Look-Alike Mother-Daughter Interview
In March 2026, I attended a production of the musical play Hairspray. The lead role of Tracy Turnblad — the plump teenager whose dream of dancing on television was finally realized — was brilliantly played by 25-year-old actress, Evie Becerra. After the play, I entered the lobby and caught sight of ‘Tracy’. I went over to her to congratulate her profusely on her fine performance. However, the woman I had approached was Tracy’s 51-year-old mother, Monica. Their physical resemblance was extraordinary, as shown in Figure 1.
Monica (L-mother) and Evie Becerra (daughter). This is their favorite picture of themselves together, taken at the Gem Theater, in Garden Grove, California, March 14, 2026. Photo Credit: Nancy L. Segal.
They agreed to an interview that took place over Zoom on April 9, 2026. Excerpts from that interview, edited slightly for clarity, are presented below. Monica is indicated by (M), Evie is indicated by (E), and I am indicated by (NS).
NS: Do people confuse you the way I did if you’re not together?
E: Yes, mostly at shows or in situations where they see one or the other beforehand. Usually, it’s seeing me before my mom, but never my mom before me.
M: I’ve gone to California and at every show it happens — my favorite one, though, was at Grease. These three guys came up to me after the show and they were like, ‘Oh my God! You were amazing! We just couldn’t keep our eyes off of you.’ And I looked at them and I said, ‘Why, thank you.’ It went like that. And then they said, ‘Wait, but how did you get your makeup off so fast?’ And, and then I said, ‘Oh my gosh, that’s not me. It was my daughter. Wow.’
NS: Has it ever happened that someone sees you first, Monica, and thinks you are Evie?
M: It’s never gone me first. I guess that’s because they see her on stage, you know, and from the stage, you can’t tell an age … We have such, similar I mean, like exactly the same features and our round face and everything. So, I think that’s why.
NS: Evie, how do you feel when people confuse you?
E: It’s happened since I was little, so it’s not really confusing me — I’ve known my whole life that we look exactly alike. And I think it’s because the older I get, obviously, the more I’m starting to look like my mom. So, I think that’s why it’s happening a lot more often than it did. I know, it’s so crazy. It’s so funny … I think I just feel neutral about it. I mean, like I said, because it’s such a normal thing. I don’t have any wild or like different reaction to it. It’s just kind of neutral.
NS: Monica how do you feel when people confuse you?
M: It makes me feel happy because I guess I’m not aging as fast as I think I am. You know, because she’s still 25 and I’m 52. So, it’s kind of like, oh, well, thank you … I’m proud that she looks just like me. You know, it just makes me happy.
NS: Monica, how would you describe your relationship with Evie?
M: So, I fly out to almost every show. I follow her to her auditions — some people may think that I’m a stalker, you know, but I’m supportive, you know? I mean, we’ve always had a really great relationship … And I always say, as long as she’ll let me still be there, then I’m going to be there.
NS: Evie how would you describe your relationship with your mother?
E: We’ve always been really close. We’ve had a good relationship. It’s funny because when I was much younger, I was a daddy’s girl. But as I got older, I very much connected more to her side … I’m just completely thankful that I’m able to have her as a mom.
Interestingly, Monica and Evie named more personality differences than similarities; however, both admitted that they work very hard at their different jobs — Evie as an actress and Monica as a personal assistant. Of course, parents and children share 50% of their genes, whereas MZ twins share 100%, so parent-child pairs should show relatively less personality resemblance. Importantly, Monica and Evie’s extraordinary physical similarity — despite their 25-year age difference — has not interfered with their close relationship. This response has been voiced by other look-alike pairs and should help to quell identity and stigmatization concerns over possible HRC.
In closing, it is recognized that new medical procedures can be unsettling until they have been extensively tested and applied. For example, when IVF was introduced in the UK in 1978, and in the US in 1981, many people worried that this signaled the destruction of family relationships. However, IVF has not destroyed families; rather, it has allowed countless childless couples to have children of their own. Perhaps not surprisingly, IVF is also becoming a reproductive option for gay couples (especially males), who have relied solely on adoption for many years. Again, I believe the challenges raised by HRC are significantly diminished with reference to MZ twins. My hope is that other naturally occurring human models, like Monica and Evie, will contribute to this conversation that is certain to come up in the future. Nevertheless, mitigating the challenges raised by HRC does not mean that it should be utilized because many issues, currently known and as yet unknown, will require resolution prior to implementation.
Research Reviews
Infantile Pyknocytosis in a Dizygotic Twin
Infantile pyknocytosis (IP) is a rare cause of hemolytic anemia in which the red blood cells are destroyed faster than they can be made (Johns Hopkins Medicine, 2026). The source of this condition is unclear, but a genetic factor has been suspected. A case of IP was detected in the second-born member of a dichorionic diamniotic (DCDA) twin pair, delivered at 36 weeks, 2 days gestation, following labor induction due to maternal preeclampsia (Spoorenberg et al., Reference Spoorenberg, Wachters-Hagedoorn, van Wijk and de Kok2021). Both twins were female. The twins were classified as DZ, but the methods by which their zygosity was determined were not provided — approximately one-third of MZ twins are DCDA, so it is not possible to assign twin type on the basis of fetal membranes (Segal, Reference Segal2017). It was, however, noted that the affected twin’s blood type was identical to that of her mother. Extensive genetic testing of the affected twin did not reveal mutations that could be linked to IP. It was concluded that a genetically based predisposition to IP, associated with other genetic mutations, could result in IP in the event of environmental or intrinsic triggers.
MZ Twin Discordance for Hemimicrencephaly
I came across a reference to MZ twins discordant for hemimicrencephaly (HME) in a New Yorker magazine book review of Beyond Inheritance (Groopman, 2026), authored by science journalist, Roxanne Khamsi (Khamsi, Reference Khamsi2026). HME is a relatively rare malformation of cortical development marked by considerable cerebral asymmetry. I identified what I believed to be the source of a case of discordant twins referenced in the review, although it was not provided. I believe the source was an article by Salamon et al. (Reference Salamon, Andres, Chute, Nguyen, Chang, Huynh, Chandra, Andre, Cepeda, Levine, Leite, Neder, Vinters and Mathern2006), describing pathological correlates of HME in children — it was noted that one of the affected children had an unaffected MZ co-twin. An intriguing finding was that cerebral hemispheric asymmetry with contralateral HME, while detected among the nontwin children, was also evident in the twins — the twin with HME had a larger affected and smaller nonaffected cerebral hemisphere, relative to their twin sibling. Unfortunately, the methods for classifying the twins as MZ were not indicated.
Unusual Dental Development in a Twin, with Expert Commentary
On March 19, 2026, I received the following email from a twin (T) and forwarded it for comment to a dental expert studying twins (TH). I believe the exchange (edited slightly for clarity) will be of interest to readers of TRHG.
T: As I grew up into my teens it came that I was missing my two eye teeth and two wisdom teeth, and my sister was found to have four eye teeth and six wisdom teeth. Of course, this had made a good story for many years. Then I became a friend with a person who was in an OGBYN office and when I had told her about my story of my twin having my missing teeth, and she told me that we were ‘fraternal identical twins’, she then explained how it was possible. That the egg had split before it was fertilized, that we were identical on our mother’s side and fraternal on the father’s side. Have you heard of this type of twins?
TH: Semi-identical twins are incredibly rare, and they are hypothesized to arise from the fertilization of a single egg by two sperm, rather than cleavage of an egg prior to fertilization. Regardless, I don’t think it would explain the phenotype. Development of the dental lamina, which proceeds tooth development embryologically, normally doesn’t begin until 3.5 weeks post fertilization, so oocyte splitting couldn’t mechanistically influence presence or absence of specific teeth. Even conjoined twins wouldn’t be expected to have discrepant numbers of teeth due to oocyte cleavage.
Tooth number and type are highly genetically conserved, but there is still significant variation in specific teeth due to early disturbances in morphogenetic fields regulating their development. There are also thresholding allometric effects of size at the extreme ends … bigger individuals have bigger teeth, and sometimes more, and smaller individuals have smaller teeth, and sometimes less. Normally, even fraternal twins are likely to be similar in dental presentation because of (partially) shared genetics and environment in utero.
Lateral incisors (eye teeth) and third molars (wisdom teeth) are the most commonly missing teeth … and the latter are also relatively common to find as extra teeth (i.e., fourth molars), probably as a consequence of allometry. The fact that these two twins had phenotypes in the opposite direction in terms of tooth number is suggestive of a more fundamental (genetic) error in embryological regulation of tooth development … maybe a homeobox gene defect of some sort, which led to a less stable/regulated developmental trajectory, and hence a hyper-variable phenotype in these individuals for those teeth that generally show more variability anyway? This is strengthened by the fact that the wisdom teeth are the last to develop chronologically … they only start to develop at about 7–12 years of age, and over a significantly longer trajectory than the other teeth as well.
Twin Study of ‘Gaze Fingerprint Signatures’
A study of ‘gaze fingerprint signatures’ (GFS) in a nontwin sample found individual differences in how people ‘look at the world’ (Crockford et al., Reference Crockford, Satta, Severino, Fiacchino, Vitale, Bertelsen, Busuoli, Mandelli and Lombardo2026). The stimuli were 700 pictures of complex natural scenes. Interestingly, individuals with autistic traits showed reduced gaze fingerprintability for social scenes. In addition, duration of gaze was found to be nonrandom over short and long timeframes. In speculating as to the source of the interindividual variation in GFS, the authors cited a related twin study (Kennedy et al., Reference Kennedy, D’Onofrio, Quinn, Bölte, Lichtenstein and Falck-Ytter2017). The sample included 119 MZ twin pairs and 114 same-sex DZ twin pairs who were 11 years in age. It was reported that genetic factors affected the twins’ eye movements that, in turn, affect visual exploration and fixation patterns. Interestingly, correct matching of individuals based on their performance was highest for MZ twins (27.9%), followed by DZ twins (6.7%), and nontwins (.84%).
Human Interest
Identical Twin Executives at Odds
Identical twins, Allan and John Block, are central figures in their family’s business, Block Communications, Inc. (BCI). BCI is a privately owned, 118-year-old media company launched by the twins’ father, Paul Block, a German émigré. The company acquired Pittsburgh’s Post Gazette newspaper in 1926 and the Toledo Blade in 1927, with continued expansion over the years that followed (BCI, 2025). John is the company’s vice-chairman and editor of the Post Gazette; Allan is the company’s president, chairman, and CEO.
In 2023, the twins experienced a severe disagreement, ending communication between them. John wished to sell the company, while Allan vehemently opposed the idea because he believed that the timing was poor — as a result, he was summarily dismissed from his position. The issue was taken to court where it settled in 2024, with both twins resuming their former positions in the company (WTOL Newsroom, 2024). Many identical twins collaborate, given their matched interests and abilities, and most pairs do so successfully (Segal, Reference Segalin press). The case of the Block twins is exceptional, but one from which CEOs, managers, and twins can learn.
Loss of Identical Twin, Jim Whittaker
Jim Whitaker’s fame as the first American to reach the summit of Mt Everest, in 1963, is well known; moreover, Whitaker was preceded by fewer than ten people. What is not well known is that Whitaker had an identical twin brother, Lou. The 6-foot, 5-inch twins were two of the three sons born to Charles and Hortense Whitaker on February 10, 1928, in Seattle, Washington. In their teenage years, they joined the Boy Scouts, reaching the summit of Mount Rainier, the highest peak in the Cascades (Branch, Reference Branch2026). Both twins were also members of the Mountaineers, a Seattle-based climbing club.
A job at a small recreational equipment store (that eventually became Recreational Equipment Incorporated, or REI) gave Whitaker the time to develop his interests and abilities in mountain climbing. It is unclear why his twin brother did not join him for the climb up Mt Everest. However, in 1984, Lou led the first successful American expedition to the North Col of Mt Everest (Branch, Reference Branch2026). Lou passed away on March 24, 2024, at the age of 95; Jim passed away on April 7, 2026, at the age of 97 (The Mountaineers, 2024).
Identical Twin Artists
An exhibit by the Haas twins, Nikolai and Simon, opened at New York City’s Museum of Arts and Design (MAD), on April 11, 2026, and is scheduled to run until August 16, 2026. The twins appear to be fraternal, based on my inspection of photographs (MAD, 2026; Cascone, Reference Cascone2018). Their exhibit, ‘Haas Brothers: Uncanny Valley’, fuses ‘cutting-edge technology with deeply tactile, human-centered making’. The twins, who have collaborated for 15 years, capture themes of collaboration in their artistic works, also incorporated into their previous show, titled ‘Freaks’. The Haas twins’ partnership is another exception to the generally less frequent professional working relations observed among fraternal twins. Their twin relationship is another example that should highlight the ingredients of working well together.
Twin Wisdom Captured in Stone
A new book, The Whispers of Rock, by geologist Anjana Khatwa (Reference Khatwa2025), introduced readers to the Suiseki stones of Japan, arranged as objects of worship (Thorson, Reference Thorson2026). Khatwa suggests that listeners can receive spiritual messages from these stones if they are in the proper frame of mind. There is a stone she touches every morning that was excavated from the grave of her deceased twin brother in northern Minnesota — she claims to have contact with her twin in this way. This assertion sounds unusual coming from a scientist; however, it is understandable that some surviving twins choose a particular object or ritual as reminiscent of their twin. I have known twins who assume the habits of their deceased cotwin, such as adopting their manner of dress, as a way of keeping them in their own lives.
