The twin bond starts before birth — and it never fully ends. Whether twins grow up inseparable or drift apart, whether they fight through their twenties or barely speak, research consistently shows that the bond formed in the womb shapes their sense of self, their relationships, and their grief throughout their entire lives. What changes across the decades is not the bond’s existence but its form.
We’ve covered it before including writing about the twin bond in 2004, in Twins Magazine’s Guide to the First Year. That piece was addressed to new parents. Twenty years on, what’s changed?
What Our 2004 Coverage Said
Patricia Malmstrom’s “The Twin Bond,” published in our Guide to the First Year, addressed a question that most new twin parents arrive at within days of bringing two babies home: How will I bond with both of them at once? Her answer was warm and direct — bonding is a process, not a single event. It unfolds gradually through the daily repetition of feeding, soothing, learning who each baby is. For further reading, see Is it OK to have a favorite.
The twins themselves, she noted, were already ahead of their parents. They had been bonding with each other since before birth — and early research was beginning to suggest that newborn twins were a genuine comfort to each other, that the presence of one could improve the other’s health. She ended with a prediction that proved more complicated than it first sounded: “They are individuals born as a team… their bond brings with it life-enhancing possibilities seldom enjoyed by singletons.”
Malmstrom was writing about the bond’s beginning. What she could not cover — because her readers’ twins were still in arms — was where it goes.
What the Science Now Shows
The science of twin bonds across a lifetime has grown substantially since the early days of researching it. The most important update: the bond is not a fixed state. It moves through predictable stages, and the nature of those stages is shaped as much by how twins were parented as by whether they are identical or fraternal.
Dr. Barbara Klein, psychologist and twin researcher whose clinical work spans more than thirty years, describes twins as essentially “born married” — sharing a deep primary attachment from the womb, communicating in ways that can seem opaque to outsiders, and drawing on each other for a sense of stability that persists across the lifespan. Forming close relationships with non-twins, Klein finds, remains genuinely difficult for many twins at any age. The comfort they learned to take from each other early creates a template that nothing else quite matches, and can lead to intense separation anxiety whenever that closeness is disrupted.
Klein’s longitudinal research with adult twins identifies two distinct identities that twins carry simultaneously: a twin identity, which generates the deep attachment, and an individual identity, which generates competition, frustration, and the drive for separateness. These two pulls exist in permanent tension. The twin relationship, she writes, is a source of wonder, contentment, loneliness, and anxiety all at once — and twins, unlike married couples, cannot divorce.
“Twins have two distinct identities — as a twin and as an individual. Twin identity creates the deep attachment that twins long to share throughout their lifespan. Individuality creates competition, anger, and resentment.” — Dr. Barbara Klein, Psychology Today
Her research with 30 sets of adult twins found that the bond type established in infancy tends to hold for life. Twins raised as individuals — with their bond respected but their separateness actively encouraged — showed the most adaptive outcomes as adults. Twins raised as a unit, or pitted as opposites, showed the most friction. The implication for parents is pointed: the bond you’re nurturing right now is likely the bond your twins will carry for the rest of their lives.
The teenage years introduce predictable turbulence. Identity questions about careers, friendships, and independence pull twins in different directions. Anger becomes more visible, and if communication breaks down, estrangement becomes possible. Then comes early adulthood — the stage Klein describes as the most crucial, where loyalty and betrayal enter the relationship in new ways, and where repair finally becomes genuinely possible.
What Malmstrom Got Right
The core of Malmstrom’s 2004 piece holds. Her instinct that “even pairs who have a feisty relationship will come to each other’s defense” is confirmed by decades of clinical observation. Klein’s research describes it precisely: anger toward a co-twin lives side by side with deep love and a longing that never really disappears. The fighting and the closeness are not opposites — they are the same relationship expressed differently.
Malmstrom’s observation that twins are “individuals born as a team” also proves durable. What she was naming in 2004 is exactly the dual twin identity Klein’s research maps clinically: the permanent negotiation between self and pair that twins navigate from the womb onward. The tension doesn’t resolve. Adults learn to manage it — or they don’t, and the relationship pays the price.
Her prediction about “life-enhancing possibilities” is real too, but it warrants the honest addition she couldn’t have included when her readers’ twins were newborns: those possibilities come at a cost. Being a twin in a non-twin world is harder than the popular image suggests. Klein’s research found that adult twins frequently avoid appearing in public together because it triggers the same intrusive questions they faced as children — who’s smarter, who’s nicer, where’s your other half — questions that reduce two individuals to a spectacle. The twin bond is a gift and a weight, simultaneously, for most twins who carry it into adulthood.
What This Means If You’re Raising Twins Now
The research points in one consistent direction: the most important thing parents can do for the twin bond is also the most important thing they can do for each twin individually. Treat them as distinct people. Encourage separate experiences. Resist the pull to manage them as a unit, even when it’s easier. Klein’s decades of clinical work suggest that twins raised as individuals maintain both a close bond and their independence — the best of both outcomes.
That doesn’t mean forcing separation or discouraging closeness. The bond is real and valuable and doesn’t need to be dismantled. It means ensuring that each twin has experiences, friendships, and a sense of self that exist outside of the twinship — so that when the inevitable turbulence of adolescence and early adulthood arrives, they have the internal resources to navigate it without losing each other.
The 20s rupture, which appears again and again in the accounts of adult twins, is not a failure of the bond. It is the bond being stress-tested. Most twins who go through it come out the other side closer than they were before — different people, but still a team. If you’re a twin family navigating this stage right now, you’re not watching something break. You’re watching something grow.
“Young twins who were very close and interdependent remained very psychologically attached to one another as adults. The bond, once established, endures.” — Dr. Barbara Klein, Not All Twins Are Alike
Is the twin bond stronger in identical twins than fraternal twins?
Not necessarily. Dr. Barbara Klein’s research with 30 sets of adult twins found that the strength and quality of the twin bond was determined primarily by parenting — how much twins were raised as individuals versus as a unit — not by whether they were identical or fraternal. Both types can share a very close bond or a very conflicted and distant one.
Why do so many twins drift apart in their twenties?
The twenties are when twins typically establish independent adult identities for the first time — different careers, partners, cities, social circles. This individuation is normal and healthy, but it can feel like a rupture, especially for the twin who is more dependent on the relationship. Klein’s research describes this as a predictable life stage, not a failure of the bond. Many twins who drift apart in their twenties reconnect in their thirties with a closer, more mature relationship.
Does the twin bond affect romantic relationships?
It can. Klein’s research describes twins as longing to recreate the quality of closeness and mutual understanding they have with their co-twin in their romantic partnerships. Partners who understand the twin bond and don’t feel displaced by it tend to report the least friction. The complications arise most often when one twin’s life revolves primarily around the twinship, which can leave partners feeling like a permanent third wheel.
What happens to a twin when their co-twin dies?
Twin bereavement is recognized as a distinct category of loss. Surviving twins can experience a sevenfold elevated risk of psychiatric difficulty in the months following their co-twin’s death, with risk remaining elevated for years. Klein describes the twinless twin as someone who may feel that part of themselves has been permanently lost — a grief that affects personality structure for the rest of life. Support that specifically acknowledges the nature of the twin relationship makes a significant difference.
Can twins who have become estranged repair their relationship?
Klein’s clinical research suggests that adulthood is actually the most realistic window for repair. Once the intensity of adolescent identity battles has passed and both twins have established independent lives, the conditions for reconciliation improve significantly. The anger that drives estrangement, she observes, almost always coexists with a deep longing — the two emotions are rarely as separate as they appear from the outside. For further reading, see Same DNA, Different People.
