Practice at closeness from day one
Twins go through everything at once: the same teachers, the same shows, the same awkward year. That gives many of them an early, easy sense of what it means to share a life with someone. The same is true, to a lesser degree, of any siblings born very close together. In a relationship, a twin may be comfortable with constant contact, quick to share and unbothered by a partner being around all the time.
Whether twins really differ in adult personality because of it is hard to say. Twins are studied heavily for what they reveal about genes, but much less for how they love, so this is drawn from what twins and their partners report.
Wanting to be one person
The other side is identity. Twins are often compared, mixed up or treated as a unit, and many spend their teens and twenties carving out separate tastes, friends and cities. Some identical twins feel this most strongly. A partner who sees them clearly as themselves, not as “one of the twins,” can matter a great deal.
Dating someone who has a twin
Expect the twin to be in the picture. They may text daily, know things before you do, and have a shorthand you can’t follow. That bond rarely leaves less room for you, but it can feel like it. Saying so kindly, and building your own rituals as a couple, tends to help more than competing. If you’re the twin, letting your partner in on the shorthand helps too, and so does saying out loud that a plan with your twin isn’t a vote against your partner. Some couples set aside a standing evening for the twin, so it stops feeling like a surprise each time.
Twins and the rest of the sibling line
A twin can also be an eldest, a middle or a youngest relative to other siblings, and those patterns can sit alongside the twin one. A pair of twins with a much younger sibling may have grown up as joint firstborns, sharing the job of looking after the little one. Twins born after an older sibling may carry some youngest child ease, competing more with each other than with the eldest.
There’s no rule about which birth order suits a twin best. Popular claims that twins should date other twins are untested. The birth order overview has more on where these ideas come from.