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(an) only child
一人っ子なので兄弟がいない。
Since I am an only child, I have no siblings.
一人っ子で育った人はマイペースな傾向がある。
People who grew up as only children tend to go at their own pace.
一人っ子は親から多くの期待をかけられることが多い。
Only children are often burdened with high expectations from their parents.