1. Assigning Roles
It's impossible not to compare twins with one another. For example, one of my twins is an extrovert. He's always smiling, laughing, and engaging with people. The other twin is an introvert. He likes to smile at people from a distance, all the while sucking his thumb and holding his blankie. He's very friendly, but not in the aggressive way of his brother.
What we need to remember as parents is that we shouldn't cast our extroverted twin in the role of "the friendly one" and the introverted twin in the role of "the shy one". It isn't fair to either twin, and could squelch the possibility of their developing other traits as they get older.
A more extreme version of role-casting happens when one twin is cast as the "smart" or "good" twin and other is cast as the "dumb" or "bad" twin. These roles can come about when one or both parents identify more with one twin than the other, or when one twin has a more pleasant personality type than the other. These types of roles can damage both twins, in that unrealistic expectations are placed on both, and their relationship with each other is also diminished because of resentment and competition.
2. Being too "Fair"
Even identical twins sometimes need to be parented differently. When misbehaving, one twin may need a parent to take a more firm tone of voice, while the other may find such a tone of voice distressing to the point of tears. When they're older, one twin may respond better to time-outs, while another may respond better to having privileges taken away. Parents need to take time to figure out how to best discipline each twin, and discipline each according to his/her individual needs.
3. Allowing Twins to "Divide and Conquer" Parents
Twins have been learning to cooperate since they started to crawl. One way they often cooperate is in the manipulation of their parents. The places where you and your spouse disagree or differ in parenting techniques are often the areas where twins try to "divide and conquer." Take this scenario, for example.
Twin A and Twin B are at the store with their parents. Mom has said if they are good, they can buy some candy. The twins misbehave. Mom says, "OK, no candy, you two." The twins run to their dad, crying and apologizing, begging for the candy. "See, they're sorry," he says, handing them each a candy bar. Mom is furious.
The only way to avoid "divide and conquer" tactics from children is to be a united front as parents. Always back up your spouse, even if you totally disagree with what's going on. Fight about it later, when the twins can't hear you. If they know your weak spots, they will exploit them. It's human nature.
4. Discouraging Separate Identities
While it's not as common as it was in the past, some parents still see their twins as a "unit", not as two separate individuals. Naming twins with alliterative or rhyming names, dressing them the same, and referring to them as "the twins" instead of by their individual names are all ways that parents and society minimize the individuality of twins.
To encourage individual identity formation, allow each twin to be him or herself. Don't always enroll both in the same sport or activity; encourage each to have his or her own friends. As they get older, let them have separate birthday parties if they like. Spend time alone with each twin, and let him or her talk about interests, events, and ideas. Find out what makes your twins unique, and celebrate their individuality.
5. Allowing One Twin to Dominate the Other
Though most twin sets have a dominant twin, it is not wise to allow that twin to dominate to a point that the other twin's self-esteem is minimized. Dominant twins need to learn not to boss the other twin around, not to physically bully the other twin, and not to manipulate the other twin. The more passive twin needs to learn how to stand up for him or herself, and/or how to avoid situations that allow domination to occur. The trick, for parents, is to teach these skills without making the dominant twin look like a "meanie" and the passive twin look like a "victim"; that's why it's best to start this when they are toddlers, and domination involves tug of wars over toys, shoving matches, or an occasional thrown block. Teaching twins how to treat each other with respect is an 18-year project, but one well worth the effort.