A study analyzing 109 children aged 3–6 in Wuhan, China reveals two twists hidden behind the familiar claim that “children with siblings are more socially skilled.”
Introduction: “What If My Only Child Can’t Read the Room?”
Parents raising an only child often hear, or quietly entertain, a particular worry: “They say kids who grow up jostling with siblings are quicker to read situations and other people’s feelings — what if my only child misses out on that?”
The worry has some basis. A child growing up with siblings negotiates, fights, makes up, deceives, and gets fooled every single day. Research has suggested this process nurtures theory of mind — the ability to understand that others hold thoughts, beliefs, and intentions different from one’s own. Theory of mind is the capacity to reason about what’s inside someone else’s head — “Mom doesn’t know there’s a pencil in the box, so she’ll think it holds cookies” — and it underpins empathy, friendship, and understanding deception.
But are only-children really at a disadvantage here? And if so, is it a difference that can’t be undone? A 2021 study by a team at Central China Normal University answers both questions honestly — and overturns the conventional wisdom twice.
The Core Research Question
The team asked three things:
“Do children with siblings really have better mind-reading ability than only-children? And if so, can the only-child gap be closed through training?”
In detail:
- Among young children, which comes first — simple (first-order) false belief or complex (second-order) false belief?
- Do children with siblings score higher on theory of mind than only-children?
- If only-children who lag in theory of mind receive “cognitive verb” training, does their ability improve?
Method: Split Into Two Studies
The research had two parts of different character. One was a “comparison,” the other an “intervention (training).”
Study 1 — Comparing only-children and children with siblings
109 children aged 3–6 were split into three groups: first-borns (the eldest, with a younger sibling), second-borns (the youngest, with an older sibling), and only-children. Theory of mind was measured with two classic tasks.
- Unexpected content task: The child is shown a cookie box, which is then opened to reveal a pencil inside. The child is then asked, “What will a friend who’s seeing this box for the first time think is inside?” To answer correctly, the child must understand that although they know it’s a pencil, someone who doesn’t know still thinks “cookies.”
- Unexpected location task: One puppet puts a ball in a box and leaves; while it’s gone, another puppet moves the ball. Where will the returning puppet look for the ball? Answering “the original spot” shows the child has read that puppet’s (mistaken) belief.
Study 2 — Training the lagging only-children
28 only-children aged 4–5 who had failed the false-belief tasks were selected, and half (the experimental group, 15) were shown a short animation. The video naturally embedded cognitive verbs — words pointing to mental states, like “think” — within true- and false-belief scenarios. Once a week, 10–15 minutes, for just two weeks. The other half (the control group, 13) watched no video. Theory of mind scores were measured again before and after.
Study Design and Participants
- Study 1: 109 children aged 3–6 in Wuhan, China (60 boys, 49 girls) — 36 first-borns, 36 second-borns, 37 only-children
- Study 2: 28 only-children aged 4–5 who had failed the false-belief tasks — 15 experimental, 13 control
- Theory of mind scored 0–2 points per task
- Study 2 also measured language ability (Peabody Picture Vocabulary Test, PPVT-4)
Results: Conventional Wisdom Overturned Twice
Easy false beliefs first, hard ones later
Children solved simple first-order false beliefs (mean 1.05) significantly better than complex second-order false beliefs (mean 0.63). Understanding “someone else doesn’t know” is easier than understanding “what one person thinks about another person’s thoughts” — the expected developmental order.
First twist — the ones who benefited were first-borns, not all children with siblings
Controlling for age, first-borns scored significantly higher on theory of mind than only-children. So far, this fits “having siblings makes a difference.”
But second-borns showed no difference from only-children. Despite also having a sibling, the youngest gained no advantage. This even contradicted the team’s hypothesis. In other words, what built mind-reading ability was likely not simply “having a sibling,” but the position of being the eldest — teaching and caring for a younger one. The team suggested the youngest may have lacked opportunities for meaningful interaction with an older sibling, or that large age gaps limited their exchanges.
Second twist — the only-child gap closed after just two video sessions
The most striking result is in Study 2. When only-children who had failed the theory-of-mind tasks were shown videos containing cognitive verbs for just two weeks (once a week):
- Experimental group: 0.27 → 1.00, a large gain
- Control group: 0.46 → 0.23, no change (a slight decline, if anything)
- The post-training difference between groups was statistically significant
The only-child “deficit” in theory of mind was not an innate limitation but a gap that closes quickly once the right language experience is provided.
Why Would “Words” Build Mind-Reading?
The most interesting point in this study is that the key to lifting theory of mind was neither siblings nor play, but “language that points at the mind.”
Cognitive verbs like “think,” “believe,” “know,” and “don’t know” refer to the invisible contents of someone’s head. By hearing and using these words, a child gains a linguistic handle for grasping the abstract idea that “each person’s mind holds different contents.”
An analogy: in a home with siblings, phrases like “Your brother doesn’t know you hid it” or “I didn’t realize your sister would think that” fly around countless times a day. It’s an environment naturally overflowing with mind-pointing language. An only child’s home may have relatively fewer such exchanges. If so, what’s missing isn’t “a sibling” but “the experience of putting minds into words” — and parents can supply that all on their own.
Practical Implications
No need to feel anxious just because your child is an only child
This is the study’s most important message. Even if an only child’s theory of mind lags slightly on average, it was a gap closed by a brief language experience. The stigma of “a child who lacks social skills for want of siblings” rests on weak ground.
Use “mind-pointing words” often in daily life
“Mom didn’t know you were upset,” “Your friend thought it was theirs,” “Grandma doesn’t know you’ve arrived, so she’ll be surprised” — conversations that name the invisible mind out loud become nourishment for mind-reading ability. Asking about a character’s inner thoughts while reading a book has the same effect.
Having siblings doesn’t make it automatic, either
This study showed second-borns were no different from only-children. The mere presence of a sibling is no magic. What matters — whether from siblings or parents — is the child’s experience of bumping up against other people’s different minds and putting that into words.
Limitations
Honest notes the team acknowledged:
- Study 1 did not measure language ability. Language and theory of mind are closely linked, so the possibility that group differences arose from differences in language level can’t be ruled out.
- Only classic false-belief tasks were used. Other facets of theory of mind — emotion understanding, diverse-desire understanding — went unobserved.
- The age-gap effect for second-borns was not analyzed separately. How the age difference from an older sibling affected results is left for future work.
- The sample was limited to middle-class families in one Chinese city, and Study 2 had only 28 participants. Generalization calls for caution.
Closing
What builds a child’s mind-reading ability is not “siblings,” but the experience of putting an invisible mind into words.
Having siblings simply makes such experiences naturally more frequent — but being an only child doesn’t close off the path. This study hands only-child parents a concrete handle instead of anxiety. Reading with your child today and asking once, “What do you think this character is thinking right now?” — that small conversation opens the door for a child to step into another person’s mind.
Source: Zhang, Z., Yu, H., Long, M., & Li, H. (2021). Worse theory of mind in only-children compared to children with siblings and its intervention. Frontiers in Psychology, 12, 754168. https://doi.org/10.3389/fpsyg.2021.754168