High 3D:5D ratio: A possible correlate of externalizing and internalizing problems. An exploratory study

Background and Objectives: The second to fourth (2D:4D) digit ratio is a sexually dimorphic trait which has been studied to examine the association between fetal hormones and a variety of behaviors. Lower 2D:4D ratios, suggestive of exposure to higher levels of prenatal testosterone, have been associated with male-linked disorders, while higher 2D:4D ratios, suggestive of exposure to weaker prenatal androgen action, have been associated with female-linked disorders. Past research has concentrated on the 2D:4D ratio, whereas the relationship between other ratios, such as the 3D:5D ratio, and psychopathology has not much been studied before. Therefore, the aim of this study was to assess the correlation between the 2D:4D and 3D:5D ratio, and internalizing as well as externalizing symptoms, in a large non-clinical sample (143 boys, 150 girls) of white Caucasian children aged 7 to 13 years. Methods: Externalizing and internalizing symptoms were assessed with the Child Behavior Checklist (CBCL). Results: The 3D:5D ratio in boys and in girls was positively associated with scores on Externalizing Problems. Further, in girls only, the 3D:5D ratio was positively correlated to scores on Internalizing Problems. Conclusions: The 3D:5D ratio can be considered a correlate of externalizing and internalizing problems in children from the general population. Received: 9 April 2010 Revised: 24 September 2010 Accepted: 18 October 2010 HIGH 3D:5D RATIO: A POSSIBLE CORRELATE OF EXTERNALIZING AND INTERNALIZING... 47 In humans, the critical period in prenatal life for sexual differentiation starts around the 7th-8th week after conception1. Until this moment the genitalia are undifferentiated and the level of testosterone determines whether the fetus then starts to develop into male or female. Prenatal hormones that influence the sexual differentiation also have an effect on sexual differences in behavior. Fetal hormones and its effects on behavior can be studied in a variety of ways. Direct measurements such as hormone levels being extracted from the umbilical cord at birth or assessing hormonal levels of the fetus or the mother during pregnancy suffer from disadvantages due to their invasive nature, selection biases, and ethical considerations. Researchers interested in the effect of prenatal hormones on the behavior of children or adults, therefore often rely on indirect measures. Cohen-Bendahan and colleagues2 described three indirect methods of examining the behavioral effects of prenatal exposure to sex hormones: otoaucoustic emissions, finger length ratio, and dermatoglyphics. In this paper, we will focus on finger length ratios. Since 18753, it is known that the length of the index finger, compared to the length of the ring finger, the 2D:4D ratio, is a sexually dimorphic trait, not only in humans4 but also in bonobos5. Lower ratios are more common in men and ratios closer to one are more characteristic of women6,7. The relative length of fingers is fixed for life within the first three months of pregnancy6,8. Finger length ratio is considered as a marker of the amount of testosterone the fetus was exposed to in the womb. A higher level of prenatal testosterone is associated with more male-like behaviors, and with a lower 2D: 4D ratio, and vice versa, a high 2D:4D ratio is associated with lower levels of fetal testosterone6,9. McFadden and Shubel10 showed the largest sex differences in 2D:4D, 2D:5D and 3D:4D, but also 3D:5D differed significantly between the sexes. And more recently, it was found that across different studies, the 2D:5D was the most and the 2D:3D was the least sexually differentiated11. Since 1875 many association studies with the 2D:4D ratio have been carried out (currently a 2D:4D digit ratio search in five scientific databases leads to 757 hits, as compared to 22 hits for a 3D:5D digit ratio search). A large number of studies have related the 2D:4D ratio to a variety of variables (i.e., assertiveness, breast cancer, fertility, hand preference, homosexuality)4,12. More recently the 2D:4D ratio has been associated with psychiatric traits, personality, and social behaviors that show sex differences13-15. However, for other finger length ratios, such as for instance the 3D:5D ratio, hardly any studies are available. Since McIntyre et al.16 found that among children, comparison with 3D distinguishes the sexes much better than with 2D and McFadden and Bracht17 found that the largest effect sizes were demonstrated in ratios that involved the 5th digit, we considered the 3D: 5D ratio a logical choice of ratio besides the more commonly studied 2D:4D ratio. Therefore the topic of this paper is to assess the association between the 3D:5D ratio, and psychopathology, more specifically, internalizing and externalizing symptoms. With respect to digit ratios, prenatal testosterone, and the association with externalizing behavior, some evidence stems from studies in patients with congenital adrenal hyperplasia (CAH). These individuals were exposed to high levels of testosterone in the womb. Later in life they exhibit more masculine digit ratios and externalizing behavior than normal controls18,19. However, results across studies are inconsistent because for instance Buck and colleagues20 did not find masculinized digit ratios in patients with CAH. 48 ESTHER I. DE BRUIN ET AL. Further, Stevenson et al.21 found associations between a low 2D:4D ratio in female college students and symptoms of AttentionDeficit/Hyperactivity Disorder (ADHD). Similarly, Martel et al.22 showed that a masculinized 2D:4D ratio was correlated with ADHD symptoms in boys. Although they did not study the 3D:5D ratio, the authors assessed the 3D:4D ratio, and found a similar negative association with ADHD symptoms. De Bruin et al.15 showed that boys with ADHD or Oppositional Defiant Disorder (ODD) had significantly lower 2D:4D ratios than boys with anxiety disorders, who had a more female-like ratio. Further, a lower 2D:4D ratio was found to be associated with more aggression in men13. As for the 3D:5D ratio, Stevenson et al.21 found a weak positive correlation between the right hand 3D:5D ratio and ADHD symptoms in males. Further, McFadden et al.23 showed that boys with ADHD actually differed more from control boys on the 3D:5D ratio (and on the 2D:5D, and 4D:5D ratio) than on the more common 2D:4D ratio which only showed small differences across groups. With respect to the association of digit ratios and internalizing problems, it was found that men with a higher 2D:4D ratio reported higher levels of anxiety24, and that depression in men was associated with more female-like digit ratios14. So far, no studies of the association between anxiety, depression, and the 3D:5D ratio have been found. Summarized, previous studies have mostly focused on the 2D:4D ratio and it is therefore unknown whether the 2D:4D ratio can be considered a specific indicator for psychiatric traits. Previous studies recommend to study other ratios as well, and a currently no information is available about the possible correlations between the 3D:5D ratio and psychopathology in the general (child) population. Therefore the aim of this study was to assess the association between internalizing symptoms such as anxiety and depression, externalizing symptoms such as aggression, on the one hand, and the more common 2D:4D ratio, as well as the less explored 3D:5D ratio, on the other, in a large sample of children from the general population. Since the 3D:5D ratio follows the same sexual dimorphic pattern as the 2D:4D ratio, albeit to a lesser extent, we expected to find associations in a similar direction. Thus, we expected a lower 3D:5D ratio (and 2D:4D ratio) to be associated with more externalizing problems, and a higher 3D:5D ratio (and 2D:4D ratio) to be associated with more internalizing problems.