Research Proposal 2D:4D and Perceived Aggression: a Human Subjects Study Maya Frost-Belansky. Introduction

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1 Research Proposal 2D:4D and Perceived Aggression: a Human Subjects Study Maya Frost-Belansky Introduction The ratio of second digit (index finger) to fourth digit (ring finger) or 2D:4D is a sexually dimorphic trait that is smaller for males than for females (Bailey and Hurd, 2005). This finger length ratio is fixed by age two and it is posited that it remains stable through puberty into adulthood (Manning et al., 1998). 2D:4D is negatively correlated with levels of fetal testosterone relative to fetal estradiol (Lutchmaya et al., 2004). Thus, smaller 2D:4D ratios are thought to be associated with higher prenatal testosterone levels and/or greater sensitivity to androgens (Bailey and Hurd, 2005). 2D:4D has been found to be related to characteristics such as aggression in men, male dominance perceived by women, physical fitness, and prevalence of childhood ADHD (Bailey and Hurd, 2005; Neave et al., 2003; Hönekopp et al., 2006; Roberts and Martel, 2013). In their 2005 study, Bailey and Hurd found that 2D:4D was negatively correlated with physical aggression in men. In addition, men with lower 2D:4D ratios were rated as more masculine and dominant by women who were shown photographs of their faces (Neave et al., 2003). With each successive birth, levels of prenatal testosterone decline (Maccoby et al., 1979). Interestingly however, very few studies concerning 2D:4D (if any at all) have controlled for birth order. It seems rather prudent to consider birth order because 2D:4D is a direct result of prenatal testosterone and birth order significantly influences prenatal testosterone levels. I will examine whether or not birth order plays a role in finger length ratio, and if it does, I will control for it in my investigation of 2D:4D and perceived aggression. 1

2 In this study, I will test the hypothesis that high plasma testosterone levels yield greater sensitivity to provocation and threat (Olweus et al., 1980). This hypothesis allows for the critical prediction that individuals with lower 2D:4D ratios (i.e. more prenatal testosterone) will perceive aggression at a lower intensity than individuals with higher finger length ratios because they are more sensitive to confrontational behaviors. Similarly, this could be a case of projection, in which the individual projects an unacceptable characteristic they sense in themselves onto another individual as a coping mechanism (Freud and Strachey, 1964). This concept also supports the prediction that 2D:4D would be positively correlated with level of perceived aggression. On the contrary, it is possible that individuals with lower 2D:4D ratios (i.e. more aggressive) simply find aggressive and confrontational behaviors to be the norm and therefore will not sense aggression until it becomes more intense. To test the above hypotheses and predictions, I will provide students at Fairview High School with a continuum of faces and prompt them to decide at which numbered face the facial expression changes to aggressiveness (see Appendix A). I will then compare this with the individual s 2D:4D ratio. Because 2D:4D is a sexually dimorphic trait, males and females will be considered separately. Only students aged will be included in the study, and since 2D:4D is thought to be stable after age two (Manning et al., 1998), age should not affect the accuracy of finger length ratio measurements. Furthermore, emotions conveyed through facial expressions have been found to be universal across cultures (Ekman, 1971), and thus cultural differences should not influence the level at which an individual perceives aggression. 2

3 Methods Sample Collection. I will be present at all sample collections and will supervise the subjects throughout the entire process. Each subject will be provided with a Continuum of Faces sheet (see Appendix A), a Subject Information Sheet (see Appendix B), and a diagram instructing subjects on how to properly measure their fingers (see Appendix C). The facial expressions get progressively more aggressive as the number gets higher. I created the faces using a program developed by Spindler and Fadrus in In addition, I will have each subject mark the lengths of the index and ring fingers of both right and left hands in the appropriate box on the subject information sheet (see Appendix B). I will then measure the lengths with a with a precision of ±0.5 mm. The accuracy of this method is discussed below. In addition, I will ask participants to indicate their gender, whether or not they are a child of a multiple-birth, whether or not they are adopted and/or aware of their birth order in regards to their biological mother, and their birth order. A study done on the accuracy of self-reported 2D:4D via the Internet explains that while self-reported values tend to be higher and sexual dimorphism is weaker, having large sample sizes will counteract the measurement error that reduces effect sizes (Caswell and Manning, 2009). I plan on having a sample size of upwards of 400 individuals, which is much larger than those of most studies concerning 2D:4D. I will conduct a brief study to test the accuracy of this method in which I will ask 16 of my fellow Science Research Seminar students to record their finger lengths using the methods above and then I will measure those lengths with a ruler measuring with a precision of ±0.5 mm. I will then measure each individual s finger length ratios with Vernier calipers with a precision of ±0.5 mm. 3

4 Statistical Analysis. To determine if there is a relationship between birth order and 2D:4D, I will run a linear regression. If a significant correlation is found between birth order and 2D:4D, I will run a multiple linear regression with birth order and 2D:4D as the independent variables and perceived aggression as the dependent variable. If no significant correlation is found between birth order and 2D:4D, I will run a linear regression on 2D:4D and perceived aggression because I can assume that birth order does not effect 2D:4D and it does not need to be controlled for. Subjects Subject Selection. Fairview High School is located in Boulder, Colorado. The school requires that all students take at least one year of biology, and thus biology students are an acceptable sample population. Other independent factors such as age and culture are controlled for and discussed in the introduction. 2D:4D is a sexually dimorphic trait, so males and females will be examined separately. Risks and Safety. There are no risks associated with this study as it is entirely anonymous and the experiment is both physically and psychologically non-invasive. The only personal information that is requested of the participants is gender, birth order, and whether or not they are a child of a multiple-birth, none of which can be used as identifiers. Consent Form. As discussed above, there is very minimal risk involved in this study and so it is unnecessary to require participants or their parents to sign informed consent forms. 4

5 Literature Cited Bailey, A. A., & Hurd, P. L. (2005). Finger length ratio (2D: 4D) correlates with physical aggression in men but not in women. Biological psychology, 68(3), Caswell, N., & Manning, J. T. (2009). A comparison of finger 2D: 4D by self-report direct measurement and experimenter measurement from photocopy: methodological issues. Archives of sexual behavior, 38(1), Ekman, P. (1971). Universals and cultural differences in facial expressions of emotion. In Nebraska symposium on motivation. University of Nebraska Press. Freud, S., & Strachey, J. E. (1964). The standard edition of the complete psychological works of Sigmund Freud. Hönekopp, J., Manning, J.T., & Müller, C. (2006). Digit ratio (2D: 4D) and physical fitness in males and females: Evidence for effects of prenatal androgens on sexually selected traits. Hormones and Behavior, 49(4), Lutchmaya, S., Baron-Cohen, S., Raggatt, P., Knickmeyer, R., Manning, J.T. (2004). 2nd to 4th digit ratios, fetal testosterone and estradiol. Early Human Development, 77, Maccoby, E. E., Doering, C. H., Jacklin, C. N., & Kraemer, H. (1979). Concentrations of sex hormones in umbilical-cord blood: their relation to sex and birth order of infants. Child Development, Manning, J. T., Scutt, D., Wilson, J., & Lewis-Jones, D. I. (1998). The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Human reproduction, 13(11), Neave, N., Laing, S., Fink, B., & Manning, J. T. (2003). Second to fourth digit ratio, testosterone and perceived male dominance. Proceedings of the Royal Society of London. Series B: 5

6 Biological Sciences, 270(1529), Olweus, D., Mattsson, A., Schalling, D., & Loew, H. (1980). Testosterone, aggression, physical, and personality dimensions in normal adolescent males. Psychosomatic medicine, 42(2), Roberts, B.A., & Martel, M.M. (2013). Prenatal testosterone and preschool Disruptive Behavioral Disorders. Personality and Individual Differences, (55)8, Spindler, O., & Fadrus, T. (2009). Grimace project documentation. Vienna University of Technology. URL: 6

7 Appendix A Continuum of Faces *Note: the actual Continuum of Faces sheet displays the images shown at twice the size. This will allow participants to see the subtle differences between sequential faces. 7

8 Appendix B Subject Information Sheet Please circle the option that applies to you. Gender: M F Are you a twin, triplet, etc.? Yes No If you re adopted and unsure of your birth order from your biological mother, skip the following question. Birth Order: 1 st 2 nd 3 rd 4 th 5 th Other Beginning at Face #1, at which numbered face do you feel that the facial expression changes to aggression? If you feel that Face #1 is displaying aggression, write the number 1. If you feel that none of the faces display aggression, write the number 10. In the appropriate box below, turn the correct hand so that your palm is face up. Put the crease on your finger that is closest to your palm on the bottom line. Make sure your finger is parallel with the edge of the paper. Mark a straight line directly above the tip of your finger. Repeat this process with the other three fingers making sure that you re using the correct finger and hand in each box. See instruction page for a detailed figure. Left Hand- 2 nd (Index) Finger Left Hand- 4 th (Ring) Finger Right Hand- 4 th (Ring) Finger Right Hand- 2 nd (Index) Finger 8

9 Appendix C Instructions for Hand Placement Left Hand- 2 nd (Index) Finger The proper finger and hand are being used in this box Mark is straight and is directly above tip of finger Finger is parallel to edge of paper Palm is face up Lower crease between finger and palm is on the line 9