Many parents, especially mothers, feel that children of multiple birth should be dressed alike and share the same friends and engage in the same play activities. This is particularly true when the children are of the same sex. Being subjected to pressures to be alike and being denied opportunities to develop their individualities leaves its mark on their personalities and on their patterns of behaviour.

Below is the list that briefly explains some of the most frequently reported characteristics of twins. It should be apparent that hereditary factors are by no means alone responsible for these characteristics; environmental factors, especially in the form of social pressures, play an even more important role. As Koch has concluded from her extensive studies of twins, “It does seem to me that the play of forces, biological and social, upon twins is rather different in many respects from that which molds singletons”.

Developmental Lag

In physical, mental, motor, and speech development, twins tend to lag behind singletons of the same age. They lag in motor and speech development. This lag may be due to brain damage or to prematurity but it is more likely to be due to parental overprotectiveness.

Physical Development

Twins tend to be smaller, age for age, than singletons. This is generally due to the fact that they are premature. They also suffer from brain damage and other physical defects more often than singletons.

Mental Development

Mental similarities between identical twins are much greater than between nonidentical twins and this persists into old age. Identical twins also show strong similarities in terms of special abilities, such as musical and artistic aptitudes.

Social Development

Twins tend to compete for adult attention, to imitate each other’s speech and behaviour, and to depend on each other for companionship during the preschool years. As they grow older, sibling rivalry and competition develop. One twin usually takes on the role of leader, forcing the other into the role of follower. This affects their relationships with other family members and with outsiders.

Personality Development

Many twins have difficulty in developing a sense of personal identity. This is especially true of identical twins and of non-identical twins of the same sex. Others enjoy the close relationship of twinship and the attention they receive as a result of their similarity in appearance. This leads to self-satisfaction and self-confidence.

Behavior Problems

Behavior problems have been reported to be more common among twins than among singletons of the same ages. It is thought that this is a result of the way twins are treated, both at home and outside the home. Behavior problems have also been reported to be more common among non-identical than among identical twins. It has been suggested that this is because rivalry is stronger between nonidentical than identical twins.

Long-Term Effects of Twinship. To date, few longitudinal studies of twins have been made. Those that have been made rarely go beyond the tenth year of the twin’s lives though cross-sectional studies of twins at different ages have been more frequent. These studies have indicated the following long-term effects.

There is a tendency for the developmental lag in physical development to end before children reach puberty and often much earlier. Generally the firstborn twin continues to be larger, brighter, and better adjusted socially throughout the childhood years. The smaller the twins at birth, the longer the developmental lag tends to persist.

The mutual dependency or “twinning relationship” so common among young twins and the one-sided dependency of the smaller on the larger twin generally give way to social relationships similar to those of singletons before the twins enter school. Those who attend day-care centers or preschools tend to abandon these patterns of dependency earlier than twins whose environment is limited to the home. Koch has reported that fraternal twins are more vulnerable to external pressures and to have less support from the twinship relationship than do identical twins not only when they are young but also as they grow older. A study of the syntactic abilities of identical and fraternal twins and their siblings has shown that by the time twins are five years old, these abilities are as good as those of their siblings.