In 2012, I wrote Alone In The Mirror: Twins In Therapy. I have received so many many phone calls from twins who cannot seem to get along with one another as adults even though they are successful in their careers and have their own children and families. Huge screaming matches and personally antagonistic and hateful fights—sometimes even physical fights—are common. I have given these deeply troubling fights some thought and here is what I have come up with.

Adult twins fight with each other when they are not completely psychologically separated from one another. Saying this differently, adult twins still get confused about who is who. Well into adulthood, commonly they see themselves in the other twin and want their twin to be more like them.

As an example, your twin brother or sister gains some weight. The less-weighty twin hates that their twin brother or sister has gotten bigger, because they still feel a part of their twin, who now does not fit into their sense of themselves. If twins don’t look the same—maybe they are no longer twins.

In my own life, my twin sister was appalled when I became pregnant because I looked fat—and not like her. She found my difference from her very troubling in so many ways that she avoided being with me. Other twins who I have talked with share very similar experiences. As my sister and I got older and she had her own children, it was hard for both of us to not criticize the other’s parenting style. My sister was way more open-minded than I was, which created unspoken anger at one another.

My examples follow into different situations and circumstances. Differences that can be enlivening and freeing for some twins can create anger and disappointment between some adult twins, who are in most cases high-functioning reasonable people.

Fighting between twins is a sign of the blurring of boundaries, whether emotional or physical, that confuse the identity of each individual twin.

I know this sounds confusing but if you are a twin you will probably understand. The best way to handle anger with your twin is to reflect on how you are different. Try to enjoy your twin for who they are. Avoid wishing your twin were different and that you were in charge of changing them.