Sibling position is part of the basics of systemic counselling — eldest daughter, middle child, youngest. Walter Toman dedicated an entire theory of partner selection to this observation in 1961, and Frank Sulloway developed a theory of personality formation in 1996. Three decades later, large-scale studies have shown: The popular theses barely withstand empirical scrutiny. This does not render sibling position worthless for practice — but it changes how it may be used.

Toman’s Complementarity Hypothesis

Austrian-American psychologist Walter Toman published "Family Constellation" in 1961, a work still cited in systemic training literature today. Toman’s central thesis is elegant: From one’s position in the sibling sequence — birth order, gender of siblings, age gaps — stable interpersonal patterns emerge. An eldest brother of sisters typically develops different relationship expectations than a youngest sister of brothers. From this, Toman derived the complementarity hypothesis: Partnerships and friendships are more successful when sibling positions are complementary — the older brother of sisters finds in a younger sister of brothers a familiar constellation and thus relationship stability.

The theory has clinical plausibility, is vivid, and structures conversations. In systemic textbooks, the Carl-Auer Lexicon, and a broad counselling literature, Toman is still referenced — often without the crucial caveat that empirical support is thin. Toman himself developed his hypotheses based on clinical impressions and non-randomised surveys; his validation studies remained methodologically limited. In the five decades since his book appeared, the necessary large-scale studies on sibling position and partner choice have either not been published at all or have yielded negative results — a fact that has surprisingly little unsettled a field that had already adopted the idea.

Sulloway’s "Born to Rebel"

Frank Sulloway, a historian of science at Berkeley, published "Born to Rebel" in 1996 — an ambitious book arguing from historical datasets that birth order has a massive impact on personality. Firstborns are said to be more conservative, conscientious, less open to new ideas; laterborns more rebellious, open, and more likely to be agents of scientific and political revolutions. Sulloway supported this with historical case analyses and an evolutionary-psychological narrative: Laterborns must carve out their own niches, not having to compete for parental attention already claimed by established firstborns — hence the "rebel" phenotype.

The book became popular, the approach widely discussed — and from the outset, personality research viewed it with scepticism. Methodologically, Sulloway’s material suffered from the selectivity of historical sources, retrospective attribution of personality traits, and the well-known weakness of between-family comparisons for birth order effects (children of different birth ranks almost always grow up in different families with different conditions — a confound that can only be controlled for by within-family comparisons). Sulloway’s thesis has since held the status of an elegant but unconfirmed hypothesis within personality psychology.

The Replication Crisis: Damian, Roberts and Rohrer

Two major studies from 2015 have decisively shaped the empirical debate — and not in favour of the strong Toman/Sulloway theses. Rodica Damian and Brent Roberts (2015, Journal of Research in Personality) analysed data from over 377,000 US high school students and found no substantial personality differences between firstborns, middle children, lastborns, and only children. While statistically significant effects were detectable given the sample size, they were so small as to be irrelevant for clinical or counselling practice — on the order of a few percent of a standard deviation point.

Julia Rohrer, Boris Egloff, and Stefan Schmukle (2015, PNAS) replicated these findings in three large European datasets (Germany, UK, USA, totalling over 20,000 adults) — this time using within-family comparisons, which are methodologically far superior. The result was consistent: No substantial effects of birth order on the Big Five (extraversion, agreeableness, conscientiousness, neuroticism, openness). Only one small effect remained robust: firstborns show a slight intelligence advantage of about 1 to 1.5 IQ points over laterborns — likely explained by parental investment patterns and intellectual resources in early childhood. But at the level of personality: nothing of the magnitude Sulloway had claimed.

These two studies are so methodologically robust and large-scale that they can be considered the state of the art. Toman and Sulloway are thus not "falsified" in the strict Popperian sense — a particular sub-hypothesis could still apply in a specific population under specific conditions. But the powerful, popular generalisations are not empirically tenable. Anyone who interprets sibling position deterministically in counselling — "You are the eldest daughter, so you are over-responsible" — is making a claim not supported by the evidence.

What Remains? Heuristic, Not Determinism

Does this mean sibling position has no place in the genogram? On the contrary — it belongs there, but in a different function than Toman and Sulloway envisaged. Sibling position is a useful conversational anchor, not a diagnostic finding. The question "What role did you have as the eldest of three sisters in your family?" opens a space for reflection in which the client can sort out for themselves which tasks, expectations, and self-images were linked to this position. The result is not "because eldest daughter, therefore over-responsible", but "in our particular family, being the eldest meant this and that for me — and the following was an implicit expectation I fulfilled without ever consciously deciding to".

The shift is subtle but important: Sibling position is a contextual factor that interacts with specific family dynamics — not a fate determined by mere position. An eldest sister in a family with a chronically exhausted mother and three younger children will indeed often be parentified; an eldest sister in a resource-rich family with clear generational boundaries will be less so. Toman’s observations are not wrong, they are simply much more context-dependent and weaker than his theory claimed.

For genogram work, this leads to a pragmatic recommendation: Record sibling position, mark it visibly, address it in conversation — but as a hypothesis, not as a diagnosis. Phrases like "I have the impression you may have taken on a role in your family that could be linked to being the eldest daughter — how do you see that?" are methodologically sounder than "As the eldest daughter, you typically...". The client decides whether the hypothesis fits; the practitioner tests their assumptions in the specific case, rather than presuming them.

Special Constellations

Some special constellations, though empirically and clinically under-researched, are practically important and can give birth order real significance: Large age gaps (seven years or more) often create quasi-only-child conditions for each child — typical sibling dynamics do not occur. Half-siblings, step-siblings, and blended family constellations complicate the position; a biologically youngest daughter may suddenly become the eldest child in a new stepfamily. Death of a sibling — especially an older one — shifts the surviving children’s family position in ways that are much more powerful than the original birth order. Here, sibling position becomes a contextual factor of genuine clinical relevance, without needing Toman’s general theory to see this.

Practice Vignette

A coach is working with a 36-year-old client, eldest of three sisters, recently promoted to department head. The client describes herself as "overly responsible", "can’t delegate", "feel like I have to save everyone". An older generation of coaches might have interpreted this, Toman in hand, as: typical eldest daughter, firstborn script, that’s your position. The younger coach holds back. Instead, she asks openly: "What specific roles did you take on in your family — and which of those roles might now actually be ready to end?"

In their work together, it emerges that the client had indeed taken on the role of relieving her depressed mother early on — not because she was the eldest daughter, but because the family configuration made this particular person available for that function. A younger sister in the same family could have been given the same task if the age gaps had been different. The coaching process gradually dissolves the internalised roles — not through birth order enlightenment, but by differentiating between past circumstances and present freedom of choice. Sibling position served as an anchor, not a diagnosis.

State of Research & Discussion

The empirical situation regarding sibling position is, unflattering as it may be for the popular counselling literature, quite clear. Damian/Roberts (2015) and Rohrer et al. (2015) have largely debunked Sulloway’s Big Five birth order hypothesis. Later replications — including in Asian and Latin American samples — confirm the picture: If effects exist, they are small and highly context-dependent. The only consistently replicated effect remains the slight IQ advantage for firstborns, the causes of which are hotly debated (confounding with parental investment, sequential resource allocation, social stimulation as the eldest child).

Nevertheless, the Toman narrative continues to be handed down in systemic counselling practice in German-speaking countries. There are two reasons for this: Clinically, the heuristic often "works" in some way, because sibling position correlates with other relevant variables (family role, parental attention, assignment of responsibility) — the practitioner "sees" patterns that are in fact produced by these correlated variables. And didactically, the Toman narrative is compact and vivid — it sells better than the more honest message "Sibling position is usually a weak predictor; check in the specific case what actually happened." Practitioners who work reflectively with Toman should therefore do so consciously — as a didactic structuring tool, not as a diagnostic algorithm.

Cross-References

• M3 "Bowen Differentiation" — Bowen’s concept of sibling position operates on a different level (triangulation dynamics), not with Toman-style personality typology.
• M4 Lesson 12 — in career coaching, sibling position can become relevant, but the same caution applies.
• M4 Lesson 17 — Reliability: Genogram statements require empirical humility, and sibling position is a good example of this in practice.

Further Reading

• https://www.pnas.org/doi/10.1073/pnas.1506451112 (Rohrer et al. 2015, PNAS — the methodologically rigorous large-scale study)
• https://www.sciencedirect.com/science/article/abs/pii/S0092656615000379 (Damian & Roberts 2015, JRP — the US large-scale study)
• https://www.carl-auer.de/magazin/systemisches-lexikon/genogramm (systemisches Lexikon — as an example of the ongoing transmission of the Toman heuristic)
• https://de.wikipedia.org/wiki/Geschwisterforschung (Overview, with research status)