Beyond 2D:4D — AGD, OAE, and Other Markers of Prenatal Hormones
2D:4D is only one of several indirect markers researchers use to estimate prenatal androgen exposure. Other markers — AGD (anogenital distance) and OAE (otoacoustic emissions) — have also been studied. Each marker differs in measurement method, reliability, and effect size. This article compares the major markers and explains why 2D:4D remains the most commonly used despite not being the most theoretically reliable.
1. Why use multiple markers?
Prenatal hormone exposure cannot be measured directly in humans. Some amniocentesis sample data exist, but ethical and practical constraints are heavy. Researchers therefore use post-natal physical traces that correlate with prenatal hormones as "markers." Multiple markers allow cross-validation.
2. AGD — the most direct marker
AGD (Anogenital Distance) is the distance from the anus to the base of the genitals. In animal experiments, AGD is the morphological feature most directly tied to prenatal androgen exposure. Male rats exposed to androgens show longer AGD; those given androgen blockers show shorter AGD.
Similar patterns appear in humans. Swan et al. (2005) reported that newborn male AGD was inversely correlated with maternal phthalate (an environmental hormone disruptor) exposure, suggesting AGD reflects prenatal androgen environment in humans as well.
AGD has practical limits. It is highly invasive (genital-region measurement) and difficult to collect at scale in adult samples. So while AGD is regarded as "theoretically the cleanest marker," its use is mostly restricted to neonatal, infant, and clinical samples.
3. OAE — a hormone signal you can hear
OAE (Otoacoustic Emissions) are very faint sounds emitted from the cochlea, either spontaneously or in response to stimuli. They are measured with a specialized microphone in the ear canal and are routinely used for newborn hearing screening.
The interesting feature is the strong sex difference: female OAEs are clearly stronger than male, with an effect size larger than that of 2D:4D. Additionally, McFadden (1993, 1998) reported that homosexual women show slightly weaker OAEs than heterosexual women — supporting the interpretation of OAE as a marker of prenatal androgen exposure.
OAE's strength is measurement precision: rater-to-rater variation is essentially zero compared to fingers. Its weakness is the need for equipment, plus the fact that external factors affecting hearing (noise exposure, certain drugs) can confound the result.
4. Other markers
Less prominent markers include:
- Other digit ratios: 3D:4D, 2D:5D, etc. 2D:4D became the standard because it shows the largest sex difference.
- Facial sexual dimorphism: jaw-to-forehead ratios and similar metrics reflect both prenatal and pubertal hormones. Not a pure prenatal marker.
- Body fat distribution: waist-to-hip ratio (WHR) and similar measures correlate with hormone environment but are dominated by post-puberty effects.
5. Comparing markers — which is most reliable?
A rough academic ranking:
- AGD: highest theoretical directness. Rarely used in adult studies due to measurement difficulty.
- OAE: highest measurement precision; large effect size. Requires equipment.
- 2D:4D: easiest to measure. Moderate sex-difference effect size. Smallest behavioral correlations.
Studies that measured 2D:4D and OAE in the same individuals found weak correlations between them (r ≈ 0.1–0.3). If both reflected the same prenatal hormone signal, a stronger correlation would be expected. The weak overlap suggests each marker carries its own noise on top of the hormone signal.
6. Why does 2D:4D remain the most popular?
Despite weaker theoretical reliability than AGD or OAE, 2D:4D dominates the literature because of measurement accessibility. A caliper, scanner, or smartphone is enough; even self-measurement datasets are usable. This makes large samples easy to assemble and statistical power easy to achieve.
That accessibility cuts both ways. Many good studies become possible — but so do low-quality samples and publication bias. Many of the criticisms raised by Voracek and others trace back to this trade-off.
7. Conclusion
2D:4D is the easiest prenatal-hormone marker to measure, but not the most reliable. AGD is theoretically cleaner; OAE is more precise. Yet none of these markers, used alone, gives a signal strong enough to predict an individual's personality or behavior.
If you received a result from this service, treat the number as "the most easily measured of several weak hormone signals." A different marker on the same person might paint a slightly different picture.
Key References
- Swan SH et al. (2005). Decrease in anogenital distance among male infants with prenatal phthalate exposure. Environmental Health Perspectives, 113(8).
- McFadden D (1998). Sex differences in the auditory system. Developmental Neuropsychology, 14(2-3).
- McFadden D, Pasanen EG (1998). Comparison of the auditory systems of heterosexuals and homosexuals: Click-evoked otoacoustic emissions. PNAS, 95(5).
- Hines M (2004). Brain Gender. Oxford University Press.