Abstract

This paper investigates whether academic rank, academic field, and gender account for the pay disparity in higher education in the United States. Analyzing 2,235 faculty in the University of Iowa, I find that pay gaps are primarily driven by academic rank, especially among professors and non-tenured faculty. Male assistant professors earn 23.8% more than females, while non-tenured males earn 31.6% less than females. Within identical academic ranks, there are gender pay gaps between assistant professors and non-tenured faculty. By analyzing 301 assistant professors, this paper identifies the academic field as another factor in pay discrepancies across academia, particularly among business, medical, social science, and STEM disciplines. However, gender doesn’t contribute to the pay disparity problem when faculties are under the same academic rank and field of study. Given this paper does not utilize datasets for any private institutions or colleges in other states, the paper’s findings can only be generalized to public universities in the U.S.  

I. Introduction 

In August 2023, five of Vassar’s female professors sued the college for wage discrimination against female faculties. According to the Washington Post, full-time male professors at Vassar earn an average annual salary of about $154,200, while their full-time female professors only earn $139,300. In this lawsuit, advocates for Vassar professors argued that the gender pay gap arose due to substantial differences in starting salaries.  In academia, there exists a merit rating system biased against females, alongside a discriminatory promotion process that systematically prevents or delays the advancement of female professors compared to males. This gender bias and stark compensation difference between male and female faculty members does not only happen at Vassar College. The American Association of University Professors (AAUP) finds that full-time women professors make up 82% of what their male colleagues earn across academia. Recent lawsuits in Vassar have alleged wage discrimination against female professors, raising questions about the presence of the gender pay gap in academia across the United States. 

The College and University Professional Association for Human Resources has discovered persistent pay disparities for females in staff and faculty positions at colleges and universities across the United States. Academic researchers and policymakers hypothesize that the gender gap in earnings persists because it is hidden intentionally (Trotter et al., 2017). Given that limited research focuses on the gender pay gap for higher education in the United States, this paper aims to provide evidence of how male and female academic faculties differ in their earnings at colleges or universities in the United States. Moreover, this paper also investigates non-gendered factors contributing to the pay gap in academia across the United States like academic rank and field.

Using the University of Iowa as its primary data source, this paper examines whether academic rank, academic field, and gender account for the pay disparity for higher education in the United States. Motivated by Koedel and Pham’s research in 2023, I categorize determinants of salary disparity into two areas: conditional gaps and unconditional gaps. Since compensation differences can be explained partially by the level of faculty’s skills and contributions, the conditional gaps include academic rank and academic field. The remaining unexplained portion of the pay gap falls under the unconditional gap like gender. 

Faculties with higher seniority typically take more responsibility in teaching and administrative tasks, have more years of experience, and potentially make more substantial contributions to research. Therefore, academic rank has the most significant impact on pay disparity in academia. Besides academic rank, the academic field is the second most influential factor. As some academic fields like medicine or business are historically more prestigious or better funded for research, it leads to a greater pay discrepancy when faculty members have identical academic ranks. As a result, I hypothesize that gender is the least influential factor in the pay gap problem. 

II. Background  

President Obama proposed the White House Equal Pay Pledge in 2016 to narrow the gender wage gap across the United States. Following Obama’s campaign, academic institutions formed new committees and commissions on college campuses to address the gender pay inequality problems. For instance, Louisiana State University established the Council on Gender  Equity to emphasize gender pay equality. 

However, research still shows a persistent gender pay gap among university faculty across the United States. AAUP finds that a full-time female professor earned roughly $82 for every $100 a full-time male professor earned in 2023. This compensation difference between male and female full-time professors raises questions about the existence of a gender pay gap for higher education in the United States. Does pay depend on the skills and contributions of males and females equally, or does bias result in differential salary solely based on the individual's gender?

From the late 1980s to the mid 2010s, previous research indicates that the gender pay gap, a form of unconditional gap, accounts for 20% of wage difference at research universities (Koedel & Pham, 2023). Besides the unconditional gap, the conditional gap contains the unexplained portion of the pay gap in academia. The conditional gap is typically associated with the academic field, years in the position, and peer performance evaluations. In contrast to the unconditional gap, the conditional gap only accounts for 4% to 6% of wage difference, or  20% to 30% of the unconditional gap (Li & Koedel, 2017). Given that the unconditional gender pay gap is almost three to five times bigger than the conditional gap, this sizable difference underscores the substantial influence of gender on salaries within academia. Furthermore, this finding also suggests that the pay gap in academia is more closely tied to gender-based disparities than to intellectual or performance measurements. 

III. Data

My main estimates are based on an analysis of data from the Iowa Legislature for the  University of Iowa in 2022. This annual census survey collects data on full-time and part-time teaching and administrative staff at degree-granting public universities and their affiliated colleges in Iowa State from July 1 to June 30 of the following year. The survey covers 14,295  employees in the University of Iowa, including administrative and support staff, librarians, all full-time tenure track faculties, part-time affiliated staff, adjunct staff, clinical staff in teaching hospitals, visiting scholars, and research staff who have academic ranks and salaries similar to teaching staff, for all those whose term of appointment is not less than 12 months. The main objective of this paper is to examine how male and female academic faculties differ in their earnings at universities or colleges in the United States. Thus, this dataset excludes clinical staff, 

visiting scholars, research staff, teaching assistants, administrative and support staff, and librarians.  

Since the Iowa Legislature mandates that state employees participate in the annual census survey, confidential pay information is obtained directly from the Department of Administrative  Services without additional verification or editing. As the publicly available salary data is directly obtained from this State Employee Salary Book, the accuracy of data and publicly accessible features make the University of Iowa an ideal choice for this paper. Data about demographics, qualification data, and salaries were collected for all tenured, tenure-track, and non-tenured track faculty. Demographic data include each faculty member’s gender and county information. Qualification data include each faculty member’s academic rank and academic field. The salary data is directly obtained from the Iowa State Employee Salary Book payroll records. 

A limitation of the Iowa Legislature database is the generalizability problem. The database only includes state-funded public universities like the University of Iowa, Iowa State  University, and the University of Northern Iowa. Other private universities or liberal arts colleges are not included in the Iowa Legislature database due to confidentiality concerns regarding wage information. This restriction poses a significant challenge in drawing broader conclusions about pay disparities across all public or private institutions in the United States.  

IV. Methods 

Given that the range of faculty’s wages varies from $1,000 to $1,685,834, this paper uses a log specification for salary to normalize the scales of the variables to make it less prone to 

outliers. To examine whether the faculty’s academic rank correlates with the pay gap, I analyzed  2,235 faculty members among tenured, tenure track, and non-tenured track at the University of  Iowa for 2022. Tenured faculty typically secure lifetime professor employment after a six-year probationary period. Tenure track faculty hold positions as Associate Professor or Assistant  Professor and are currently in the promotion and evaluation process towards attaining the status of tenured full professor. All faculties besides tenured or tenure track are classified as nontenured track faculties. The tenured faculties include 496 employees as full-time professors. The tenured track includes 415 employees as full-time associate professors or assistant professors.  The non-tenured track includes 1,324 employees as part-time Adjunct staff, Professors of Instruction, Professors of Practice, and Lecturers. The general regression model is represented as follows: 

log Salary=   β1AssociateProfessor + β2NonTenured + β3Professor + β4Male +    

β5AssociateProfessor  Male + β6NonTenured  Male + β7Professor  Male + α + ε     

where Male is an indicator variable that equals 1 if the assistant professor is a male and 0 if not.  AssociateProfessor, Professor, and NonTenured are indicator variables that equal 1 if the faculty member is associate professor, professor, or non-tenured track accordingly, and 0 if not. To test for differential returns for ranking by sex, I include three interaction variables:  AssociateProfessor Male, Professor Male, and NonTenured * Male. These interaction variables are equal to 1 if the faculty member is a male assistant professor, male professor, or male in a non-tenured track accordingly, and 0 otherwise. 

To testify whether the faculty’s academic field correlates with the pay gap, I controlled the academic rank effect and analyzed 301 assistant professors at the University of Iowa for  2022. Faculties are sampled from six academic disciplines: art, business, humanities, medicine, social science, and STEM (Science, Technology, Engineering, and Mathematics). Once a discipline was selected for sampling, all assistant professors listed on the department website were included in the dataset. The Arts discipline includes 22 assistant professors from the Arts  Division in the College of Liberal Arts and Sciences. The Humanity discipline includes 21 assistant professors from the Humanities Division at the College of Liberal Arts and Sciences. The STEM discipline includes 61 assistant professors from the College of Engineering and Natural and Mathematical Sciences Division in the College of Liberal Arts and Sciences. The Social Sciences discipline includes 41 assistant professors from the Social Sciences Division in the College of Liberal Arts and Sciences and College of Education. The Business discipline includes 23 assistant professors belonging to the College of Business. The Medical discipline includes 133 assistant professors from the College of Medicine, Dentistry, and Nursing. The general regression model is represented as follows: 

log Salary=   β1Business + β2Humanity + β3Medical + β4SocialScience + β5STEM +         β6Male + β7Business  Male + β8Humanity  Male + β9Medical  Male +                                                        β10SocialScience  Male + β11STEM  Male + α + ε

where Male is an indicator variable that equals 1 if the assistant professor in the arts field is male and 0 otherwise. Business, Humanity, Medical, SocialScience, and STEM are indicator variables that equal 1 if the faculty member is business, humanity, medical, social science, or STEM  accordingly, and 0 if not. To test for differential returns for the academic field by sex, I include five interaction variables: Business Male, Humanity Male, Medical Male, SocialScience   Male, and STEM * Male. These interaction variables are equal to 1 if the assistant professor is a male in business, humanity, medical, social science, or STEM, and 0 otherwise.

V. Results 

V.a.1 Academic Rank on Salary 

AAUP finds that the average annual salary in 2021 for assistant professors, associate professors, and professors was about $83,300, $96,000, and $140,500 respectively. Professors typically have more years of experience and research grants, so they tend to have higher salaries than associate professors or assistant professors. From this result, I hypothesize that academic rank correlates with pay disparity in academia. 

To test my hypothesis, the regression in Table 1 compared each faculty member’s salary based on their academic rank, with the expectation that professors have the highest wages compared to associate professors, assistant professors, and non-tenured track faculties. The non-tenured track faculty in my dataset consists of adjunct staff, lecturers, Professors of Instruction, and Professors of Practice. In the University of Iowa, the Professor of Instruction and Professor of Practice are instructional faculty solely responsible for teaching and not involved in administrative duties. Since the hiring criteria for instructional faculty is less rigorous in terms of research and scholarship requirements as compared to those for tenured or tenure track faculty, instructional faculty tend to receive lower compensation. 

In this regression, the adjusted R2 is 54.8%, which is consistent with my hypothesis that academic rank accounts for salary disparity. The β1, β2, and β3 coefficients indicate relative salary differences for female associate professors, non-tenured track faculties, and professors compared to female assistant professors. Because of higher ranks, the predicted signs for β1 and β3 coefficients are positive, suggesting that female associate professors or professors receive higher salaries compared to female assistant professors. However, the β1 coefficient is negative and not statistically significant at 0.01 level, suggesting pay is not increasing for associate professors. 

The β3 coefficient is positive and suggests that female Professors earn relatively 26.3% higher than female assistant professors. Since non-tenured track faculties are part-time basis or instructional faculty, they receive lower compensation than their tenured or tenure track colleagues. Thus, the predicted sign for β2 is negative. The β2 coefficient is negative and suggests female non-tenured track faculty earn relatively 173.7% less than female assistant professors.  Given both Professor and NonTenure variables are economically significant and statistically significant at 0.01 level, I conclude that academic rank accounts for wage discrepancies only in the professor and non-tenured track faculty levels.  

V.a.2 Gender Effect on Salary Within Same Academic Rank 

AAUP found that full-time women professors earned 82 cents for every dollar their male counterparts earned in 2023. This compensation disparity motivated me to investigate whether gender influences salary within identical academic rank conditions. In Figure 1, male faculty members earn much more than their female counterparts under professors, associate professors,  and assistant professors levels. However, female non-tenured faculty members receive higher salaries than their male colleagues. Therefore, within the same academic rank, I hypothesize that gender accounts for pay discrepancy. 

To test my hypothesis, the regression in Table 1 compares each faculty member’s salary based on their gender when they have identical academic ranks. The β4, β5, β6, and β7 coefficients indicate relative salary differences for male assistant professors, associate professors, non-tenured track faculties, and professors compared to their female counterparts under the same academic rank condition. I expect males to earn less at the non-tenured level while earning more at the professor, associate professor, or assistant professor level. The predicted sign for β6 is negative while β4, β5, and β7 are positive. However, both β5 and β7 coefficients are not statistically significant at 0.01 level, suggesting that the pay gap for the associate professor and professor level cannot be explained by gender reasons. In contrast, the β4 coefficient is positive and suggests that male Assistant Professor earns relatively 23.8% higher than female assistant professors. The β6 coefficient is negative and suggests that male non-tenured track faculties earn relatively 31.6% less than female non-tenured track faculties. Since both Male and NonTenure *  Male variables are economically significant and statistically significant at 0.01 level. I conclude that gender bias accounts for wage disparity only in assistant professor and non-tenured levels when both female and male faculty have identical academic ranks. 

V.b.1 Academic Field on Salary Within the Same Academic Rank 

Under the same academic rank, is gender the sole factor contributing to the wage disparity at the University of Iowa? Previous research suggests that academic rank and academic field account for 4% to 6% of wage difference in academia (Li & Koedel, 2017). To examine whether the academic field impacts wage differences, I analyzed 301 assistant professors across six academic departments at the University of Iowa in 2022. 

The regression in Table 2 compared each assistant professor’s salary based on their academic field. I anticipate that medical, business, and STEM assistant professors will have the highest wages relative to other disciplines. Professions like doctors, investment bankers, and software engineers are known for their lucrative salaries. As a result, students are more likely to declare majors in medical, business, or STEM subjects. To meet the growing demand for these majors while providing a more robust academic curriculum, universities, and liberal arts colleges offer competitive salaries to attract top-tier talent for teaching positions. Therefore, I hypothesize that the academic field accounts for wage discrepancy at the assistant professor level. In this regression, the adjusted R2 is 32.9%, which is consistent with my hypothesis that the academic field accounts for salary disparity. The β1, β2, β3, β4, and β5 coefficients indicate relative salary differences for female assistant professors in business, humanity, medicine, social science, and STEM compared to the female assistant professors in the arts field. Since the business, medical, and STEM fields provide lucrative salaries, the predicted signs and values for β1, β3, and β5 are positive and mathematically larger. It suggests that female assistant professors in business, medicine, or STEM receive higher salaries than those in the art department. Since the β2 coefficient is slightly positive and not statistically significant at 0.01 level, it suggests the salary difference for female assistant professors between humanities and art disciplines is negligible. In contrast, the β1, β3, β4, and β5 coefficients are positive, which suggests that female assistant professors in the business,  medical, social science, and STEM fields earn a relatively higher proportion of salaries than those in the art department. Given that the β1, β3, β4, and β5 coefficients are positive and statistically significant at the 0.01 level, I conclude that the academic field accounts for wage disparity for assistant professors in business, medical, social science, and STEM disciplines. 

V.b.2 Gender Effect on Salary Within Same Academic Rank and Field

In Figure 2, I observed the average annual salary between male and female assistant professors separately within the Arts, Business, Humanity, Medical, Social Science, and STEM  fields at the University of Iowa in 2022. Within the same academic field, the greatest gender gap is $110,510 in the Medical department while the smallest is only $558 in the Humanities department. Given that the gender pay gap exists among all six departments, I hypothesize that 

gender accounts for pay disparity when female and male assistant professors are within the same academic field.  

To test my hypothesis, the regression in Table 2 compared each assistant professor’s salary based on their gender when they are in the same academic discipline. The β6, β7, β8, β9,  β10, and β11 coefficients indicate relative salary differences for male assistant professors in Arts,  Business, Humanity, Medical, Social Science, and STEM compared to their female assistant professors under the same academic field condition. Since the β6, β7, β8, β10, and β11 coefficients are economically insignificant at the 0.1 level and statistically insignificant at the 0.01 level, I  find that there is not any gender pay gap under the assistant professor level within the Arts,  Business, Humanity, Social Science, and STEM departments. However, the β9 coefficient suggests the relative salary difference for male assistant professors is 32.4% higher than for female assistant professors within the medical field. As the β9 coefficient is economically significant at 0.1 level while statistically insignificant at 0.01 level, I identify that there is no gender pay gap under the assistant professor level within the medical field. In sum, gender does not account for wage disparity at the assistant professor level within the same department condition.  

V.b.3 Gender Effects for Assistant Professor in Medical Field  

Previous research suggests that women may encounter greater pay inequality in which they are underrepresented within a field (Casad et al., 2022). In Figure 3, I find 23 more male assistant professors in the Medical department, while the faculty number difference is less than  10 for each Arts, Business, Humanities, and STEM department. The economically significant relationship is present only in the Medical department due to the underrepresentation of female assistant professors. In the University of Iowa case, there are 58.6% male and 41.4% female 

assistant professors within the medical field, which implies that females are underrepresented in the medical department. As the β9 coefficient is economically significant at 0.1 level while statistically insignificant at 0.01 level, it partially confirms my hypothesis that the mismatch between male and female faculty numbers leads to the gender pay difference. 

Historical reasons point to why the medical department has more male faculty members than female. Typically, students need to take three or four years of education in medical schools along with three to nine years of medical training before they enter hospitals or academia. The Association of  American Medical Colleges (AAMC) finds that the average age for assistant professors in United States Medical Schools is 45.5 and 43.2 years old in 2023, which suggests the current assistant professors received their M.D. or D.O. degree from medical schools between 1997 and 2004. According to the AAMC records, from 1997 to 1998, 58.3% of medical school graduates were males while only 41.7% were females. From 2003 to 2004, 54.1% of medical school graduates were males and 45.9%were female. Even though the medical department still has high male representation,  the rise of advocacy for women in STEM and the increased proportion of female medical school graduates from AAMC records imply that the gender pay gap in the medical field is likely to narrow in the future.

VI. Conclusion 

I use the University of Iowa in 2022 as my dataset to investigate factors accounting for wage disparity in higher education in the United States. The findings show that academic rank explains wage differences in professors or non-tenured track faculty levels. Within the same academic rank, the gender pay gap only exists for assistant professors or non-tenured track level.  Besides academic rank, the academic field also accounts for the wage discrepancy when I limit 

my dataset to only focus on the assistant professor level. The pay gap arose among business,  medical, social science, and STEM disciplines. However, when two faculty members have identical academic ranks, there is no gender pay gap within the same department. 

To improve and expand on this research, diversifying the dataset must be a key focus by adding more public and private universities or colleges. A large dataset would provide a comprehensive perspective on whether the gender pay gap in academia is a nationwide inequality problem or a local inequality problem inside Iowa. If the study reveals stark differences between male and female faculty, it would be advisable to inform policymakers of the severity of the issue and propose equity focused policies such as implementing pay transparency laws to reduce pay inequality and associated gender gaps.

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Appendix 

Figure 1. Average Salary for Female and Male Faculties in Each Academic Rank

Figure 2. Average Salary for Female and Male Assistant Professors in Each Academic  Department 

Figure 3. Number of Female and Male Assistant Professors in Each Academic Department