Research in physical and social sciences has looked to physical and biological markers to predict or explain human behavior and performance (Alonso et al., 2018). One such observation is right hand second finger digit length relative to right hand fourth finger digit length, or second-to-fourth digit ratio (2D:4D; Manning et al., 2014; Alonso et al., 2018). The digit ratio is thought to be established during early prenatal development. Specifically, prenatal testosterone exposure is known to influence sexual dimorphism in utero (including 2D:4D) as early as the end of the first trimester (Manning et al., 2014). Unsurprisingly, therefore, females typically have a higher 2D:4D ratio than males (Manning et al., 2014; Alonso et al., 2018). Although the age at which right 2D:4D “stabilizes” is unknown, once it does it is constant throughout life (Jeevanandam and Muthu, 2016) with change generally only through structural deformation such as when trauma to the hand occurs. Additionally, prenatal testosterone exposure has organizing effects on the developing brain, and it potentially influences testosterone responsiveness later in life (Coates et al., 2009; Coco et al., 2011; Alonso et al., 2018). Consequently, it is hypothesized that right 2D:4D can be predictive of testosterone responsiveness and, by extension, certain human behavior and performance. This seems reasonable given digit ratio may be linked to androgen receptor gene structure (Manning et al., 2003; Zhang et al., 2020); and testosterone response in some high stress challenges appears predicted by a low 2D:4D (Kilduff et al., 2013a,b; though it may not be in low-stress scenarios (Kowal et al., 2020).

Testosterone is a hormone from the androgen group with cross regulation by glucocorticoid hormones, such as cortisol (Cook and Crewther, 2012). Cortisol is generally considered a stress hormone which can be arousing, however, it can also have an opposing functional outcome to testosterone; leading to increased aggressive behaviors (Papacosta et al., 2016) and in prolonged doses may also increase fatigue and anxiety (Mehta and Josephs, 2010; Cook and Crewther, 2012; Crewther et al., 2017). High levels of cortisol may also affect ability to receive and process information as well as execute motor tasks (Serpell et al., 2020b; Serpell and Cook, 2021). Due to the highly interconnected nature of these hormones they often can be examined relative to each other using testosterone to cortisol ratio (i.e., testosterone divided by cortisol; T:C; Cook and Crewther, 2012), and it has been argued that a response which promotes an increase in testosterone of greater magnitude than cortisol (i.e., increased T:C) is favorable in many contexts (Strahorn et al., 2017; Serpell et al., 2018).

Functional outcomes which may be observed with increases in testosterone and T:C can include enhanced physical work output (Smith et al., 2013; Crewther et al., 2015b; Russell et al., 2016), improved chronic training adaptation (Crewther et al., 2011), emotion change (Liening and Josephs, 2010; Cook and Crewther, 2012, 2019; Crewther and Cook, 2018), enhanced cognitive ability and memory (Celec et al., 2015; Serpell et al., 2020b), improved motor function (Serpell et al., 2020b), and enhanced competitive performance by means of increased assertiveness, and increased cohesion within a team combined with collective aggression toward opposition (Cook and Crewther, 2014, 2019; Gaviglio et al., 2014; Russell et al., 2016; Mehta et al., 2017; Crewther and Cook, 2018). The mechanisms by which these outcomes occur include activation of signaling pathways which promote mobilization of energy stores (Sato et al., 2008; Mitsuhashi et al., 2016), modulation of neuromotor units (Bonifazi et al., 2004), the accumulation of protein for muscle “growth” via mTor (Basualto-Alarcon et al., 2013), and increased motivation (Smith et al., 2013; Cook et al., 2021). Therefore, given the role testosterone can play when it comes to human behavior and performance, the mechanisms by which it works, and that prenatal testosterone exposure may influence testosterone responsiveness later in life, it is unsurprising to see that right 2D:4D has been shown to be predictive of social and competitive behaviors (Kilduff et al., 2013a,b; Crewther et al., 2015a; Nepomuceno et al., 2016), sporting performance (including in international Rugby union, distance running, skiing, and sumo wrestling; Manning, 2002, Manning et al., 2007, 2014; Tamiya et al., 2012; Kilduff et al., 2013a; Lombardo and Otieno, 2020), musical ability (Sluming and Manning, 2000), risk taking behaviors (Coates et al., 2009; Massimino et al., 2019), and academic performance (Coco et al., 2011; Tektas et al., 2019).

Although being linked to performance on many specific tasks primarily physical, it is important to note that recent research has also shown a link between 2D:4D and “good luck” but that this link was due to chance (Smoliga et al., 2021). Thus, we cannot simply associate 2D:4D as a biomarker for “nearly everything” (Smoliga et al., 2021), however, given that which was discussed in the previous paragraph it is speculatively possible that right 2D:4D can be associated with relative levels of achievement, independent of domain, context or profession where motivational and assertive behaviors are involved (Millet and Dewitte, 2008). This is because of the likely association between 2D:4D and testosterone responsiveness given the link between right 2D:4D, free testosterone and potentially androgen receptor gene structure (Manning et al., 2003). It has been hypothesized that people with low right 2D:4D are better responsive to feedback (Millet and Dewitte, 2008), and it is also pertinent to note that high levels of testosterone relative to cortisol may increase an individuals’ focus/attentiveness (Serpell et al., 2019, 2020b; Lightman et al., 2020; Serpell and Cook, 2021). Therefore, a low right 2D:4D may be common to high achievers regardless of whether they are elite athletes or specialist doctors. That is, based on right 2D:4D, and assuming effects of androgenic hormone responsiveness, it is possible that an orthopedic surgeon has similar patterns of response to a fly-half in international Rugby union.

The purpose of this work was to explore further the relationship between digit ratios and high achievement. Three small experiments were performed; firstly, we explored whether a relationship for testosterone and cortisol production with digit ratios exist in a group of professional rugby players using a relatively low-level stress to which they were highly familiar. Secondly, because testosterone and cortisol responsiveness may be related to proxy measures of receiving and processing information (e.g., cognition via pupil reflex; Serpell et al., 2019, 2020b), we explored whether digit ratios were related to pupillometry in a high performing population (also professional rugby players). Research in healthcare, military and with people with traumatic brain injury has argued increased pupil dilation (or a greater pupil constriction) is associated with increased attention; and slower onset of pupil constriction (i.e., increased pupil constriction latency) suggests delayed processing of information (Capó-Aponte et al., 2013; Thiagarajan and Ciuffreda, 2015; Lynch, 2018; Bicalho et al., 2019; Cabrera-Mino et al., 2019; McGrath et al., 2019). Finally, we compared digit ratios between varying populations (professional rugby players versus orthopedic surgeons versus political journalists). Most work exploring digit ratios and human biomarkers and functional performance is limited to right hand digit ratio only, this may be because right 2D:4D correlates better to target traits than does left 2D:4D (Manning et al., 2014). However, there is some evidence that right minus left digit ratio can predict free testosterone concentration (Kilduff et al., 2013a), and that a negative figure in this measurement can predict reactive aggression and is associated with traits more typical of males (Coyne et al., 2007). Therefore, we explored both left and right digit ratios. We hypothesized that digit ratios would not be related to testosterone reactivity in a mild familiar stress (study 1), but that we would see a relationship between right digit ratio and pupil reflex – a physical behavior which may reflect ongoing androgen responsiveness/sensitivity (study 2). Finally, we expected to see some differences in digit ratios between people of different professions (professional rugby players, orthopedic surgeons, and experienced print media political journalists) due to difference in demands of behaviors and skills for success (study 3). It was anticipated that outcomes could provide a directional hypothesis for future work exploring digit ratio and relative success and contribute to the current 2D:4D knowledge base obtained from other large population 2D:4D studies.

The purpose of this research was to explore the relationship between digit ratios and high achievement via a series of small research studies. In study one we did not observe a relationship between hormone responsiveness and digit ratios. In previous studies, under high levels of stress, this was seen (Manning et al., 2014). We deliberately chose a low intensity stressor, as indicated by an overall very low cortisol response, and at this level of stress our data suggests 2D:4D is unrelated to testosterone and cortisol response. The athletes were also highly familiar with the stress, so it posed no novelty. We suggest the 2D:4D linkage may only be observed when the stress has sufficient intensity or novelty which would be better in terms of overall conservation of response, grading the response relative to the stress intensity. However, an interesting finding was the relationships between pupillometry and digit ratios, in particular the moderate to strong positive correlation seen for right 2D:4D with left eye pupil constriction difference between direct and consensual stimulus. In previous work we argue pupillometry relates to ability to receive and process information and positively related to decline in T:C (Serpell et al., 2020b), hence declining T:C predicted poorer focus and attention. Thus, data from previous research combined with data from the present study suggest that right 2D:4D may be a marker of testosterone responsivity and therefore ability to receive and process information.

No difference in right 2D:4D was seen between print media political journalists, orthopedic surgeons, and professional rugby players. However, a novel outcome was the difference between the journalists and surgeons and rugby players for right minus left digit ratio difference, with journalists’ difference to both orthopedic surgeons and professional rugby players being significant. Previous work has linked this right and left hand 2D:4D difference to reactive aggression and masculinity (i.e., previous work has linked 2D:4D to being more “male like”; Coyne et al., 2007; Kilduff et al., 2013a). Aggression is considered a trait behavior (Serpell et al., 2020a). Journalism is a highly competitive field and often involves physical and psychological jostling for priority to stories, however, it would be hard to compare the so-called media scrum to an actual professional rugby scrum. Nevertheless, it should be considered that differences in competitive behavior of these two populations could be related to the competitive education and social role of these two figures. However, it may also be possible that successful journalism can be driven by aggression in non-physical behaviors. Australian journalists are high on extraversion and average for neuroticism (Henningham, 1997), linking to impulsive like behaviors (Perez Fuentes et al., 2016), and aggression (Bartlett and Anderson, 2012; Perez Fuentes et al., 2016).

Our results suggest that digit ratios may be indicative of androgen system responsiveness and associated behavior. Given testosterone may increase focus (Serpell et al., 2019, 2020b; Serpell and Cook, 2021), it could be argued that digit ratio is predictive of an ability to focus, a skill that would predispose to achievement in broad areas. This theory is somewhat consistent with work which has linked 2D:4D with academic performance (Coco et al., 2011; Tektas et al., 2019), and also consistent with work from our own lab which has shown that performance may be linked to testosterone via behavioral mechanisms (Cook et al., 2021). That is, digit ratio could be linked firstly to traits, and more secondary to states. These results also align more generally with other work which shows humans are a primate and performance and many behaviors can be driven, or explained, by biological or evolutionary processes (Claessens et al., 2020; Burch and Widman, 2021).

It is important that results from this study are also considered in the context of its limitations. Firstly, sample size was relatively small. However, this was an exploratory study only which was designed to help form a directional hypothesis for future work. It is worth noting, however, that other 2D:4D studies sample size range from similar to ours to hundreds, and we believe our work simply supports and grows these larger population studies. Few studies have combined digit ratio with other scientific measures (e.g., a training study measuring hormone responsiveness and pupillometry), and few have sampled professional and unique populations such as this study (professional rugby players, trained consultant orthopedic surgeons, and experienced print media political journalists). Furthermore, our statistical analysis was robust and enabled sound comparisons and relationships to be drawn. Nevertheless, we concede future work may benefit from recruiting larger samples. Though it should be acknowledged that it would be difficult to recruit large samples from these populations and apply scientific measures such as those used in this article. Secondly, although our methodology and measurement tools were robust and highly sensitive, we did not associate digit ratio to any other valid psychological or personality test; our argument that digit ratio could be linked firstly to traits, and more secondary to states may be strengthened had we. Future work may benefit from doing so. Finally, we did not recruit from a traditionally typically “artistic” population. To the knowledge of the researchers there is no known link between 2D:4D and creativity. It would be valuable to explore digit ratios in such populations. Creativity is broadly proposed as involving the ability to rapidly switch between diffuse modes and focused modes of thinking. Speculatively from a population evolutionary perspective it is easy to see the advantage of broad androgenic sensitivities, marked by 2D:4D, allowing a wide expression of talent, skills, and traits.

Research has shown that digit ratios may be reflective of performance in a number of domains. We argued that this could be because those with low 2D:4D ratios are likely to have greater androgenic sensitivity, leading to an ability of maintaining focus on certain tasks, a general feature of many achievements. Therefore, we argued that 2D:4D is likely related to trait behaviors rather than limited to a specific activity or profession. Future work would benefit from recruiting larger samples and recruiting from a greater diversity of populations including those which require increased creativity for success. It may also we useful to associate digit ratios from different populations to some valid psychological or personality test.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by University of Canberra Human Research Ethics Committee. The patients/participants provided their written informed consent to participate in this study.

BS and CC contributed equally to the design, collection and analysis of data, and manuscript preparation. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.