Beyond T: Sex-Based Differences in Athletic Performance
Beyond T: Sex-Based Differences in Athletic Performance
For anyone who has seen the 4X400-meter running mixed relay final from the World Athletic Championships in September 2019 (World Athletics) the differences in athletic performance between men and women are blatantly obvious. A woman, Justyna Święty-Ersetic from Poland, was in the lead when she took the baton for the final lap of the race with a 5 second lead over the closest competitor. A 5 second lead in a 400m running event should be nearly insurmountable. However, on this final leg of the mixed sex relay all of the other runners were men. Within 150 meters of the baton exchange Michael Cherry had passed Święty-Ersetic, and by the finish line Święty-Ersetic had fallen to 5th place. It’s not like Święty-Ersetic is some kind of second-tier athlete in the 400m; she was the 2018 European Women’s 400-meter champion and a two-time European Women’s Indoor Championship medalist in the 400-meter race (Wikipedia). To also help put the men overcoming Święty-Ersetic’s 5 second lead in perspective, in the 2023 NCAA Division 1 Men’s Outdoor Track Championships the 1st place runner finished with a time of 44.24 seconds and the 8th place runner finished with a time 45.34 seconds (Watch Athletics 1). In the 2023 NCAA Division 1 Women’s Outdoor Track Championships the 1st place runner finished with a time of 49.20 seconds and the 8th place runner finished with a time 51.12 seconds (Watch Athletics 2). So, to overcome a 5 second gap in a 400-meter race is within the expected performance difference between elite men and women. For three other men to also overcome a more than 5-second lead to push Święty-Ersetic to a 5th place finish clearly shows that elite men run considerably faster than elite women.
To anyone who follows sports, the fact that men (i.e. adult human males) run much faster than women (i.e. adult human females) is neither shocking nor controversial. Even a casual observer can see that men are generally taller, more muscular, and just overall larger than women (Roser 2019, Bassett 2020). The advantages in body height, body mass, and muscle mass provide men with inherent athletic advantages in terms of strength, power, and endurance over comparably aged, trained, and talented women (Hilton 2021, Millard-Stafford 2018, Sandbakk 2018). The anatomical and physiological factors that give men athletic advantages over comparably aged, trained, and talented women also include less visibly, but no less importantly, larger hearts and lungs, greater bone mineral density, higher hemoglobin levels, and more. Depending on the sport being compared, men outperform equally aged, talented, and trained women by anywhere from 10% to over 50%.
Now, what causes men to possess these anatomical and physiological advantages over comparably aged, trained, and talented women is a matter of some controversy. There is no question that the increased testosterone concentrations experienced by males at puberty accelerates growth in bone length and size, increases the amount of muscle mass, and overall accentuates the differences between males and females. At puberty females experience increased levels of estrogen which cause increases in the total amount of body fat and the deposition of body fat in the hips and breasts, which is not generally conducive to improved athletic performance (Bassett 2020, Hills 2010). The onset of menstruation in females during puberty, which is an essential part of female maturation, may also be associated with detrimental effects on sports performance (Meignie 2021). But are sex-based differences in blood testosterone concentrations the only factors responsible for men being bigger, faster, and stronger than women? Or might there be some inherent differences between males and females that originate at conception due to differences in sex chromosomes regardless of puberty-based differences in sex hormones? This is a controversy that is being debated not just in sports, but also in society overall.
Brief Overview of Sex Determination and Sex Differentiation
At fertilization sex is determined by the paired sex chromosomes, 46-XX for female and 46-XY for male, which lead the development of germ cells that develop into ovaries in females or testes in males (MacLaughlin 2004). Typically, the Y-encoded SRY gene causes development of male gonads, and without the SRY gene female gonads develop (Goymann 2023, Nassar 2023). Sex differentiation then occurs as the fetus develops along either the male or female pathway. Humans are sexually dimorphic with male anatomy & physiology centered around the production of sperm and female anatomy & physiology centered around the production of ova (Arnold 2019, Bhargava 2021, Goymann 2023). A major purpose of sex-differentiation is the development of male or female reproductive systems, but sex differentiation also results in sex-based differences in fetal growth with males typically being larger than females (Broere-Brown 2016, Kiserud 2017).
Undoubtedly these sex-based difference are a result of differences in gene expression between males and females. Out of 20,000 known human genes, approximately 6,500 are expressed differently in males and females (Gershoni 2017), with sex based differences in gene expression reported in more than 3,000 genes in skeletal muscle alone (Haizlip 2015). Without question some of the differences in gene expression between males and females can be attributed to differences in sex hormone concentrations, but not every difference between male and female cells is caused by differences in sex hormones (Arnold 2019, Institute of Medicine 2001). There are 27 protein-coding genes that are unique to the male specific region of the Y chromosome in humans, with some Y chromosome driven aspects of sex differentiation occurring independent of sex hormones (Skaletsky 2003).
Due to the complexity of fetal development, disorders can occur with the individual then being born with a disorder of sex development (DSD). There are a wide range of DSDs (which are beyond the scope of this paper), however, an extreme example of DSDs are those in which a person is born sexually ambiguous (aka intersex) (Witchel 2018). Fortunately, intersex conditions occur in only 0.018 percent of births (Sax 2002). Some intersex conditions may present as female at birth but have male pattern testosterone secretion due to internal testes, and these intersex conditions are often brought up as examples that human sex is not binary (Goymann 2023). However, this is disingenuous as those who are intersex are still either male or female. As stated by evolutionary biologist Richard Dawkins “Sex is pretty damn binary” (Dawkins 2023).
Brief Overview of Testosterone Concentrations During Fetal Development through Adolescence
Blood testosterone concentrations in fetal males increase considerably beginning around the seventh week of gestation when the SRY gene first initiates development of the testes. Testosterone concentrations are then elevated in fetal males from weeks 8 to 24 of gestation (Nassar 2023). Testosterone concentrations once again increase in infant males during the first five months of postnatal development in what has been termed “minipuberty” (Nassar 2023, Renault 2020). There are then typically no differences in testosterone concentrations between boys and girls until the onset of male puberty, which occurs between the ages of 9 and 14 years old with an average of 11 ½ years old (Mayo Clinic, NIH Child Health and Development). Typically, by age 14 and then throughout adulthood males will have 10-20 times higher blood testosterone concentrations than females.
The effects of testosterone are very evident during male puberty. Prior to puberty boys have about 10% more lean body mass than comparably aged girls (McManus 2011, Staiano 2012) and after puberty men have about 45% more lean body mass than comparably aged women (Hilton 2021, Millard-Stafford 2018, Sandbakk 2018). Before puberty boys are perhaps ½ cm taller than comparably aged girls (before the onset of female puberty, when girls may become briefly taller than comparably aged boys) and after puberty the average man is 12 cm taller than the average woman (Roser 2019). Of course, the higher testosterone concentrations in male puberty also contribute to the heart, lungs, and other systems of the body growing accordingly. The surge of testosterone during male puberty also causes other secondary sex traits such as deepening of the voice and growth of facial and body hair. However, it is an oversimplification to attribute all of these changes due to increases in testosterone during puberty since there are many genetic and hormonal factors (such as growth hormone) that regulate body height and body composition (Sonksen 2018).
As stated previously, the increased blood testosterone concentrations in males during puberty causes men to have anatomical and physiological attributes that give them inherent athletic advantages compared to similarly aged, talented and trained women. But is that the only time there are sex-based difference in athletic performance?
Sex-Based Differences in Athletic Performance Before Puberty
There has been a limited number of scientific evaluations of sex-based differences in sports performance before puberty. For example, Handelsman evaluated publicly available data on swimming, running, and jumping in children and adolescents and, although his figures clearly show boys aged 10-and-under running faster, swimming faster, and jumping farther than comparably aged girls, in 2017 he published these analyses in a paper titled “Sex differences in athletic performance emerge coinciding with the onset of male puberty. (Handelsman 2017)”. In 2019 Senefeld et al. (Senefeld 2019) used data from USA Swimming and reported that, before age 10, the top 5 girls swam faster than the top 5 boys but there were no differences between the swimming performance of the 10th-50th ranked girls and boys. In 2020 Huebner and Perperoglou (Huebner 2019) reported no sex-based differences in competitive weightlifting performance before age 10. To my knowledge these represent the only scholarly studies on competitive performance in children before puberty.
In contrast to the limited scholarly evaluations of children’s competitive sport performance, there are a plethora of scholarly evaluations of school-based physical fitness testing in children as young as six years old. Using tests such as the Presidential Fitness Test, FitnessGram, Eurofit Fitness Test Battery, or some other school-based physical fitness tests, it appears that boys consistently outperform girls of the same age on tests of muscular strength, muscular endurance, running speed, aerobic fitness, ball throwing and kicking distance while girls perform better on tests of flexibility. A small sampling of publications evaluating school-based physical fitness testing include a longitudinal evaluation of 240 boys and girls through ages 9-12 years old (Golle 2015), an evaluation of 85,347 fitness test results in 9-17-year-old boys and girls (Catley 2013), and evaluation of 108,295 8-year-olds (Fuhner 2021), an evaluation of 424,328 boys and girls aged 6-18 years (Tambalis 2016), an evaluation of 1,142,026 performances of a 20 meter shuttle run in 9-17 year old boys and girls from 50 countries (Tomkinson 2017), and an evaluation of 2,779,165 Eurofit performances in 9-17 year old boys and girls from 30 countries (Tomkinson 2018). Particularly interesting is the laboratory based evaluation of 688 Danish 6-7-year-old boys and girls showing that boys have more lean body mass and higher VO2max than comparable girls, and for the same amount of physical activity the boys attained higher levels of fitness (Eiberg 2005). Collectively, these studies (and many others not listed here) indicate that before puberty boys consistently outperform girls of the same age on tests of muscular strength, muscular endurance, running speed, aerobic fitness, ball throwing and kicking distance while girls perform better on tests of flexibility. While physical fitness tests do not always predict success in competition, physical fitness is often a prerequisite for success in sports.
USA Track and Field (USATF) sanctions youth track & field meets in most states as well as regional and national championship meets. The youngest age groups in USATF are the 8-and-under and the 9-10-year-old age groups, both of which can reasonably be assumed to represent pre-pubertal athletes. If we evaluate the overall youth records for the best performances in running, throwing, and jumping from USATF (USATF 2018), from the USATF National Junior Olympics (USATF 2019), the Amateur Athletic Union (AAU) Junior Olympics Records (AAU 2023), and the School Sport Australia Track & Field Championships (School Sport Australia 2016) all indicate that boys aged 10-and-under outperform girls of the same age in running distances of 100m, 200m, 400m, 800m, and 1500m, and in the shot put, javelin, and long jump by an average of 3.7% in running, 20.4% in throwing events, and 9.9% in long jump. Similarly, the age 10-and-under records from USA Swimming indicate that in 18 out of 22 events the boys’ records are faster than the girls’ by 1.8% (USA Swimming 2023). Collectively, these records of competition performance in children under age 10, who can reasonably be assumed to be pre-pubertal, show that males outperform females with only rare exceptions.
Evaluations of the overall records for best performances are often used by scholars to demonstrate sex-based differences in adult athletic performance (Hilton 2021, Millard-Stafford 2018, Sandbakk 2018). But the sex-based differences in sports performance between pre-pubertal males and females have been dismissed as being too small to be meaningful (Safer 2022, Merten2 2022), which is intriguing given the difference between finishers in swimming and track & field running often come down to hundredths of a second. It is therefore reasonable to conclude that a sex-based three percent difference in running performance between pre-pubertal males and females would indicate a profound advantage for male athletes.
Some colleagues and I have recently presented an evaluation of the sex-based differences in athletic performance before puberty at the 2023 Annual Meeting of the American College of Sports Medicine (Brown 2023). Drawing upon a national database of track and field performance (athletic.net) and evaluating the top 10 performances for boys and girls in the 8-and-under and 9-10-year-old age groups over a 5-year period, we observed that the boys consistently (and statistically) ran almost 5% faster, long jumped 6% farther, threw the shot put 20% farther, and threw the javelin 40% farther than girls of the same age. At the 2023 Annual Meeting of the American College of Sports Medicine there was another group of researchers from an entirely different university who used the same database with slightly different evaluation techniques and came to similar conclusions about the existence of male sex-based athletic advantages before puberty (Atkinson 2023).
It is therefore my conclusion that are indeed male sex-based athletic advantages before puberty. I base this conclusion upon large numbers of scholarly evaluations of school-based fitness testing, data from youth track & field records, youth swimming records, my own analysis of the top 10 national track & field performances for boys and girls in 8-and-under and 9-10-year-old age groups over a 5-year period, and a similar conclusion from another group of researchers. Yes, the sex-based differences before puberty are smaller than after puberty, but the differences are there in a pattern that results in a statistically significant difference that favors male athletes. Furthermore, small differences become extremely important in athletics as the difference between a gold medal and no medal can be fractions of a second. As there are not known to be differences in testosterone between boys and girls between the ages of 6 months old and the onset of male puberty, one can only conclude that these differences in performance are due to lasting effects of testosterone during minipuberty or are somehow linked to the Y chromosome.
What about Puberty Blockers?
Whether there are, or are not, sex-based differences in athletic performance before puberty still leaves questions about how puberty blockers affect athletic performance. And, quite simply, there is not enough information to answer questions about how puberty blockers affect athletic performance. There have not been any published studies evaluating the effects of puberty blockers on school or laboratory-based tests of muscle strength, muscle endurance, running speed, aerobic fitness, throwing or kicking distance in children. Nor have there been any studies on the effects of puberty blockers on competitive sports performance. There have been very few published studies on the effects of puberty blockers on body composition and body height.
Three long term studies on the effects of puberty blockers on body composition and body height show that the sex-based differences in lean body mass (Klaver 2018) and body height (Boogers 2022, Willemsen 2023) are not eliminated by 2 years of puberty blockers which were then followed by another 6 years of so called “gender affirming hormone therapy” (GAHT). Particularly interesting is that males who are given puberty blockers, testosterone suppression, and estrogen do not attain an adult body height that is different than would be expected for males without GAHT (Boogers 2022), and females who are given puberty blockers and testosterone attain only slightly taller adult body height than would be expected for females without GAHT (Willemsen 2023). It is undeniable that advantages in lean body mass and body height translate to athletic advantages in many sports. It is therefore reasonable to conclude that current evidence indicates that puberty blockers and subsequent GAHT are unlikely to eliminate male advantages in athletic performance, even when male testosterone concentrations are suppressed.
Sex-Based Athletic Differences in Adults
As has been well summarized and reviewed adult males typically have 45% more lean body mass and 30% less body fat than comparably aged females (Hilton 2021, Bassett 2020). Because of the greater lean body mass and corresponding muscle mass when compared to similarly aged, talented, and trained females, males have 73% higher overall muscle strength (ranging from 40 to 120% higher depending on the exercise and muscle group being compared) (Nuzzo 2023). Adult males also have a 25% higher maximal oxygen consumption (VO2max) per unit of body mass than comparable women. There are also differences between adult males and females in respiratory function, cardiac function, tendon stiffness, bone length, and pelvic shape that all provide inherent athletic advantages to males.
In adults, when compared to similarly aged, talented, and trained females, males are 10-13% faster in rowing, running, and swimming. Males outperform similarly aged, talented, and trained females by 16-22% in jumping, downhill mountain biking, and pole vaulting. Males outperform similarly aged, talented, and trained females by over 30% in weightlifting and baseball pitching speed (Hilton 2021, Millard-Stafford 2018, Sandbakk 2018). The performance differences between adult males and females are much larger than those observed in prepubescent children, with the magnified male advantages unquestionably due primarily, but not exclusively, to the increased blood testosterone concentrations experienced by males during puberty (Handelsman 2017).
While testosterone is the primary androgen, male pubertal growth is not regulated solely by testosterone and not every difference between male and female cells is caused by differences in sex hormones (Arnold 2019, Institute of Medicine 2001). Growth hormone (GH), insulin like growth factor 1 (IGF-1), sex chromosome linked genes, and many other cellular processes work together synergistically to regulate growth and development (Baron 2015, Mauras 1996, Sonksen 2018). While GH is thought to be primarily responsible for growth in children, and particularly growth in the long bones (Brinkman 2023), GH is also necessary for skeletal muscle growth (Mauras 2003). For example, in adults with growth hormone deficiency who were receiving thyroid, adrenal, and gonadal hormone replacement, administration of GH increased lean body mass by 5 kg over the course of 6 months while those who did not receive GH experienced no change in lean body mass (Salomon 1989). It is presently unclear precisely how genetics, testosterone, estrogen, GH, and IGF-1 all work together synergistically to promote growth and maturation during puberty (Gharahdaghi 2021, Sonksen 2018), but it is certainly overly simplistic to presume that male pattern growth can be stimulated solely by administering testosterone or terminated solely by suppressing testosterone.
Testosterone Suppression
There has been speculation that suppressing blood testosterone concentrations in adult males will erase male athletic advantages, thereby allowing transgender identified males (i.e. transwomen) to compete on an even playing field with females in women’s sports. The notion that suppressing testosterone in adult males will erase male athletic advantages was given the appearance of validity by the 2011 NCAA transgender inclusion policy (NCAA 2011), and the 2015 International Olympic Committee (IOC) transgender inclusion policy (IOC 2015) which state that a sufficient duration and magnitude of testosterone suppression will allow transwomen to fairly compete in women’s sports. Although there is considerable debate about whether transwomen can compete fairly in the female sport category, the evidence to date indicates that suppressing testosterone in adult males does not erase male athletic advantages.
One of the major factors contributing to male athletic advantages before and after puberty is the higher lean body mass, with corresponding higher muscle mass, exhibited by males. To date, there are 16 papers demonstrating that 6 months to 14 years of testosterone suppression in transwomen does not reduce lean body mass sufficiently to eliminate male advantages (Alvares 2022, Auer 2016, Auer 2018, Elbers 1999, Gava 2016, Gooren 2004, Haraldsen 2007, Klaver 2017, Klaver 2018, Lapauw 2008, Mueller 2011, Tack 2018, Van Caenegem 2015, Wierckx 2014, Wiik 2020, Yun 2021). Keeping in mind that adult males typically have 45% more lean body mass with a corresponding advantage in muscle mass than comparably aged females (Hilton 2021, Bassett 2020), research presently suggests than testosterone suppression in transwomen causes a 4-5% reduction in lean body mass.
Due primarily to higher lean body mass and the corresponding muscle mass, males typically have higher muscle strength than comparable females. To date, there are 8 papers demonstrating that 6 months to 14 years of testosterone suppression in transwomen does not reduce muscle strength sufficiently to eliminate male advantages (Alvares 2022, Auer 2016, Lapauw 2008, Scharff 2019, Tack 2018, Van Caenegem 2015, Wiik 2020, Yun 2021). Keeping in mind that, depending on which muscle groups are compared, adult males typically have 40-120% higher muscle strength than comparably aged females (Nuzzo 2023), research presently suggests that testosterone suppression in transwomen causes a zero to 9% reduction in muscle strength. Further demonstrating that testosterone is not the only factor responsible for muscle mass and strength in males, it has been demonstrated in males undergoing testosterone suppression for prostate cancer treatment that muscle strength and mass can be maintained, or at least the loss can be diminished, by engaging in strength training (Chen 2019, Kvorning 2006). There is no question that higher muscle strength is desirable in most, if not all, competitive athletes because of the positive relationship between muscle strength and athletic performance. Presently, research indicates that testosterone suppression does not erase male advantages in muscle size and strength.
There have been two very intriguing evaluations of the effects of testosterone suppression combined with estrogen administration in transwomen in the US Air Force (Roberts 2020, Chiccarelli 2022). These evaluations are intriguing because military personnel must meet fitness expectations, so it is reasonable to assume that the subjects engaged in regular exercise so the findings should elucidate the effects of testosterone suppression and estrogen administration in a physically active population. However, the findings are somewhat contradictory regarding whether, and to what extent, physical fitness is impaired due to testosterone suppression and estrogen administration. The first evaluation (Roberts 2020) indicates that prior to gender affirming hormones the transwomen performed 31% more push-ups and 15% more sit-ups in 1 minute and ran 1.5 miles 21% faster than comparably aged female Air Force personnel. After 2-2.5 years of testosterone suppression combined with estrogen administration the differences in sit-up performance had disappeared (indeed, the transwomen completed 2% fewer) but transwomen still completed 6% more push-ups and were 12% faster than females (Roberts 2020.). The second evaluation (Chiccarelli 2022) indicates that prior to gender affirming hormones the transwomen performed 66% more push-ups and 28% more sit-ups in 1 minute and ran 1.5 miles 18% faster than comparably aged female Air Force personnel. After 4 years of testosterone suppression combined with estrogen administration, the transwomen still performed 18% more push-ups and 8% more sit-ups in 1 minute but the difference in time to run 1.5 miles was only 0.2% faster than comparably aged female Air Force personnel (after 3 years the transwomen were still 5% faster). The discrepancy between these two studies regarding whether the differences in push-up and sit-up performance between males and females was erased by testosterone suppression combined with estrogen administration is hard to explain. Both studies (Roberts 2020, Chiccarelli 2022) used US Air Force Personnel and the same fitness testing. It’s important to note that Chiccarelli 2022 experienced considerable loss of subjects from 223 at baseline down to only 15 subjects after 4 years, which impairs the ability to draw firm conclusions from the data. Collectively, however, both of these studies indicate that testosterone suppression and estrogen administration in males will take over three years to erase sex-based differences in 1.5 mile running performance and it is unlikely that male advantages in muscular strength are erased after even 4 years.
The changes in 1.5 mile running performance with testosterone suppression and estrogen administration suggests that testosterone suppression impairs aerobic fitness (Roberts 2020, Chiccarelli 2022). However, to date, there has been only a single laboratory-based assessment of aerobic fitness in those who have undergone testosterone suppression combined with estrogen administration (Alvares 2022.). In a cross-sectional study of transwomen who had undergone an average of 14 years of testosterone suppression combined with estrogen administration, the transwomen had an absolute peak oxygen consumption that was 20% higher, peak oxygen pulse that was 17% higher, and maximal ventilation that was 17% higher than age matched women. Collectively, the data from Roberts, Chiccarelli, and Alvares suggest that testosterone suppression reduces but does not erase male advantages in cardiorespiratory function with the obvious implications for retained male advantages in endurance sports.
Finally, some of the best examples for retained male advantages in spite of testosterone suppression can be found in transwomen competing in women’s sports. A case study of an NCAA Division 1 swimmer who competed in the men’s category, then underwent two years of testosterone suppression and estrogen administration and then subsequently competed in the women’s category indicates that over 50% of the male advantage in swimming performance was retained (Senefeld 2023). Overall, for the distances of 100, 200, 500, and 1,650 yards this individual lost 4.0% percent of swimming speed when undergoing testosterone suppression, and the typical difference between men and women is 10%. As a male swimmer this individual was ranked 65th in the 500-yard freestyle and yet earned a number 1 ranking when swimming as a woman. More telling about the limited reduction in performance due to testosterone suppression is that this individual was ranked 551st as a man in the 200-yard freestyle but was ranked 3rd when swimming as a woman. A similar reduction in performance without the erasure of male advantages can be found in Cece Telfer, who was ranked 200-390th as a man, but won an NCAA Division II 400 m hurdle championship when running as a woman (Brown 2022). Another example can be found in Laurel Hubbard who was unheralded as a male weightlifter, but after identifying as a woman at age 35 and following the IOC guidelines Hubbard qualified for the Tokyo Olympics (Fair Play for Women). Collectively, evaluating three well known transwomen athletes suggest that testosterone suppression did not eliminate male advantages sufficiently to cause them to be equally ranked in women’s sports as they were in men’s, and instead they were much more successful when competing against women than they were when competing against men.
Overall, the current published peer reviewed research and evidence from a few transwomen athletes indicates that testosterone suppression in adult men for up to 14 years does not eliminate male advantages in lean body mass, muscle mass, or muscle strength. Very limited evidence provides conflicting information on the effects of 4 or more years of testosterone suppression on endurance performance with one study indicating an elimination of the sex-based differences in 1.5 mile running performance after 4 years of testosterone suppression while another study indicating retained male advantages in aerobic fitness and cardiorespiratory function after 14 years of testosterone suppression.
Summary and Conclusion
In summary, males and females differ at conception based on sex-chromosomes and genes. These chromosomal and gene differences then lead to differences in growth and development that are influenced by androgens and estrogens, but are also influenced by growth hormone (and other hormones), and the interactions of genes and hormones. There is no question that testosterone has immense effects on muscle mass, which in turn can affect muscle strength. However, current evidence indicates that it is overly simplistic to think that the use of puberty blockers, testosterone suppression, and estrogen administration will entirely eliminate male advantages in lean body mass and corresponding muscle mass, body height, muscle strength, and cardiorespiratory function sufficiently enough to level the playing field between male and female athletes.
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Gregory Brown, PhD FACSM is a Professor of Exercise Science and Director of the University of Nebraska at Kearney General Studies. This manuscript represents Dr. Brown’s opinions and is not a statement on behalf of the University of Nebraska at Kearney.
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Excellent, well written and documented. Thank you.