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Acclaimed mentor of minority mathematicians relied on tough love—but some say he went too far

Carlos Castillo-Chavez trained a record number of minority mathematicians. But his approach took a heavy toll

Illustration published on Friday, May 15, 2020.

Acclaimed mentor of minority mathematicians relied on tough love—but some say he went too far

Carlos Castillo-Chavez trained a record number of minority mathematicians. But his approach took a heavy toll

Edito's Note: This story was published/adapted with permission from Science. 

The numbers tell the story of a master mentor. Carlos Castillo-Chavez, a Mexican-born mathematical biologist, has trained some 50 PhD students, two-thirds of whom belong to groups historically underrepresented in science. He is especially proud of his “diamonds in the rough”—students from less selective undergraduate programs who have ended up with good jobs in academia, industry, and the public sector.

At Arizona State University (ASU), where he held the coveted title of Regents professor and an endowed chair, Castillo-Chavez presided over a mini-empire of programs designed to increase diversity within the mathematics community. He’s been honored by three U.S. presidents for expanding the educational horizons of thousands of minority students. His work has been fueled by nearly $50 million in grants from federal agencies and the private sector.

But that’s all in the past. On 16 May Castillo-Chavez retired from ASU. It was the final step in the dismantling of his empire.

Last August he had abruptly resigned as director of the Simon Levin Mathematical, Computational, and Modeling Sciences Center. He also gave up running two programs housed there: A graduate program in Applied Mathematics and Life and Social Sciences (AMLSS) and the Mathematical and Theoretical Biology Institute (MTBI), a summer research program for undergraduates. A majority of MTBI participants and a significant fraction of AMLSS students are from underrepresented groups. Castillo-Chavez created those programs and ran them with a rare degree of autonomy. 

Building, burning bridges 

His resignation from those posts, combined with his absence from the Tempe campus since January 2019, have been the cause of much speculation. “I was shocked when I heard,” said Ricardo Cordero-Soto, an associate professor of mathematics at California Baptist University in Riverside, who trained under Castillo-Chavez and who is also active in mentoring minority students. “I thought only death would retire him.”

Carlos Castillo-Chavez pictured in 2010 after being appointed to the President’s Committee on the National Medal of Science

ASU officials have said nothing about why Castillo-Chavez gave up his positions. “He’s gone, and he won’t be coming back,” was ASU President Michael Crow’s terse comment in a 4 March interview with The State Press, the university’s student-run newspaper. “He’s no longer in his historical role.”

But Science and The State Press have learned that Castillo-Chavez was stripped of his administrative titles only days after ASU officials resolved a complaint against him by one of his graduate students. The student, Maria Martinez, alleged in her April 2019 complaint that Castillo-Chavez maintained a “hostile work environment,” that he “participated in workplace harassment,” and that he violated federal laws protecting the rights of persons with disabilities. After a 3-month investigation, ASU officials told Martinez that Castillo-Chavez had agreed to immediately “resign from all administrative appointments at ASU” and would also be retiring.

There was no public announcement, and ASU officials did not release a report nor even write up their findings (see sidebar). But in interviews, many colleagues and former students have spoken about what they regard as a dark side to Castillo-Chavez’s passion and commitment to diversity: namely, an intolerance for disagreement and a penchant for bullying students and associates.

READ MORE: Arizona State stays mum on outcome of its investigation of workplace climate

“Carlos is equally good at building bridges and then burning those bridges,” says a mathematician who requested anonymity because of a history of contentious relationships with Castillo-Chavez. “He can work a room and come out with five people who are suddenly his best friends--until something happens that he doesn’t like. And then he turns on them.” 

“What’s sad is that Carlos did some good work,” says mathematician Wayne Raskind, a former ASU department chair who left after repeated clashes with Castillo-Chavez. “But he was allowed to go rogue. He eventually became completely full of himself and started to do some bad things. And the more he got away with, the more he did.”

Cordero-Soto says he steered one promising student away from ASU because of what he saw as Castillo-Chavez’s harsh approach to mentoring graduate students. Cordero-Soto, who is active in the Math Alliance, a national organization that promotes the mentoring of minority students, believes that “compassion” yields better results. “I tell my students that I will be their advocate and their biggest cheerleader because I’ve been there myself.” 

“I’m burned out”

The 68-year-old Castillo-Chavez says that his departure resulted from many factors that had nothing to do with the complaint. “I’m exhausted and tired,” he says. He shed his administrative duties, he says, because he was consumed with caring for his mother, who died last November after a long illness. His many off-campus commitments were another distraction, he adds. 

“I’ve been away from Arizona State for much of the last 7 or 8 years,” he says. That period includes a tumultuous 2 years at the helm of a new technical university in Ecuador, a year of cancer treatment in Boston, and most recently a visiting professorship at Brown University. “I’ve been trying to handle things from far away, and I’m burned out.”

He says he had planned to retire in December 2020 and simply moved up the date. But he acknowledges that he could have done a better job of addressing the issues Martinez raised. 

“I think her complaint had some validity,” he says. “I did not keep track of the potential seriousness of the situation. I was responsible for her [training], and I accept that responsibility.”

“Giant in the field”

The past year marks a somber end to an improbable journey that took Castillo-Chavez to the pinnacle of his profession. Born in 1952 to a working-class family in Mexico City and radicalized by the failed 1968 pro-democracy protests there, Castillo-Chavez hoped to use community theatre as a vehicle for social activism. But he pivoted to academia after losing a student acting contest.

Emigrating to the United States in 1974, he quickly earned a bachelor’s degree and started a Ph.D. program in mathematics at the University of Wisconsin at Milwaukee. However, a dinner conversation with his then-advisor and other faculty members nearly derailed his plans. The professors talked disparagingly about a university memo on its efforts to attract more minority students, he says, adding that it was clear to him they thought a Latino student wasn’t capable to making it on his own. “So I quit,” he told an interviewer for the Lathisms podcast in 2018. 

After reconsidering, he resumed graduate school at the system’s flagship campus in Madison and earned his PhD in 1984. A postdoc at Cornell led to a faculty position—and eventually, a tenured professorship--in what was then the biometrics department.

Hoping to launch others on a similar path, Castillo-Chavez founded MTBI in 1996. The 8-week summer program combines graduate-level courses with a group research project that targets a real-world problem. “What I have done over the last 20-plus years is to take students from non-selective schools and show them that their school of origin is irrelevant,” Castillo-Chavez told Lathisms.

(Graphic) N. Desai/Science; (Data) Carlos Castillo-Chavez

Of the roughly 530 undergraduates from around the world who have attended MTBI (see graphic), more than 160 have gone on to earn a PhD, he says--nearly 60% of whom are from groups historically underrepresented in science. Those graduates represent roughly 40% of all Latinx PhDs in mathematics from U.S. universities in the last decade, he says.

“Carlos Castillo-Chavez has been a giant in the field of STEM education with respect to underrepresented minorities over a period of almost two decades,” Ted Greenwood, a former program officer at the Sloan Foundation, which funded MTBI for many years, wrote in a 60th birthday tribute to him.

MTBI’s reach extended far beyond Arizona State. “Carlos began sending us alumni of his MTBI program, most of them Latino,” recalled Phil Kutzko, an emeritus professor of mathematics at the University of Iowa in Ames. “Without his trust and support, it is unlikely that the transformation of our graduate program … would have occurred,” Kutzko wrote in a recent essay about the Math Alliance, which he co-led for many years. 

“Somebody cared”

Carlos Torre, who earned his PhD under Castillo-Chavez in 2009 and has spent the past decade on Wall Street, fell under his spell as a Cornell undergraduate. “It was the first time I had a professor who looked and sounded like me,” recalls Torres, who as an infant immigrated with his family from Peru. “He had a very thick accent, and a ponytail.”

Torre was intrigued when Castillo-Chavez mentioned MTBI one day in class. He lobbied for a slot, and over the next several years he received several doses of Castillo-Chavez’s tough-love medicine. 

“He called me out when I should have been called out,” says Torre, recalling a tongue-lashing he received after once dropping a difficult graduate course that wasn’t required for his degree. As his punishment, Castillo-Chavez barred Torre from doing his own research that summer at MTBI.

Torres wasn’t happy, but he thinks that Castillo-Chavez was right. “It also meant a lot to me that somebody cared so much.”

Castillo-Chavez was recruited to ASU by Joaquin Bustoz, a beloved figure and former chair of the mathematics department who died in a car crash before Castillo-Chavez arrived. Castillo-Chavez immediately took over a summer program for minority high school students that Bustoz had run for almost 2 decades, adding it to the MTBI program as part of his portfolio for increasing diversity.

The next step, he decided, was a graduate program in applied mathematics. But Castillo-Chavez says he failed to win over his colleagues in the math department.

“Carlos had created an enormous amount of friction” in trying to obtain more resources, recalls one long-time member of the department, who requested anonymity. “And the dean decided that could not go on. So Carlos was given the opportunity to start his own program [in a different department]. There were not many people in the department who regretted seeing him leave.”

The result was the Levin center. It is located in ASU’s School of Human Evolution and Social Change, not its School of Mathematics and Statistical Sciences. (ASU operates three math departments across its sprawling campus). 

The center serves as the administrative home for AMLSS, a graduate program for applying mathematics to model how social forces affect the origin and spread of disease. Named to honor Simon Levin, a prominent theoretical biologist and Castillo-Chavez’s mentor at Cornell, the center doesn’t have its own faculty. Instead, its work is carried out by visiting scholars drawn from Castillo-Chavez’s vast network of contacts.

“Toxic” fights

Even as Castillo-Chavez’s empire grew and accolades rolled in, so did conflicts with colleagues and students. Former colleagues say senior ASU officials either ignored reports of questionable behavior or defended him because his successful track record burnished the university’s reputation.

Raskind, the former ASU math department chair, says that Castillo-Chavez maligned him in emails to top ASU officials, including Crow, and threatened to destroy his career after Raskind put in a bid for a deanship. (Raskind would later serve as dean at two other research universities.) Raskind says his offense was saying no to a request that his department provide computing resources for the new center.

Senior officials ignored Raskind’s complaints about Castillo-Chavez’ behavior, he says, “and that’s when I knew it was time to leave. I was a tenured professor, so he couldn’t ruin my career. But he tried to do similar things to some of his students, which I think was disgraceful.”

Other former and current colleagues tell similar stories. “Where it became toxic is that, whenever you would get into a disagreement with Carlos, he would make it an issue about minorities,” says one ASU faculty member. “He would send emails to everybody at the university, insinuating that you were a racist.”

One former student, who says he never saw Castillo-Chavez act maliciously, thinks that professional jealousy may have been a factor in some of those clashes. “The fifth floor [of one ASU building] was all Carlos,” says Daniel Rios-Dorio, who joined the AMLSS program in 2006 after participating in MTBI as a graduating senior from City University of New York. As many as 20 students joined the program every fall, and each got a laptop, plenty of space to work, and travel funds to attend conferences. “The man was clearly doing something right,” says Rios-Dorio, who earned his PhD in 2010. 

The AMLSS program has no prescribed set of courses and no qualifying exams. Castillo-Chavez thought such requirements might penalize students with weaker backgrounds and believed that a student’s research and publications were better metrics. But that flexibility also allowed Castillo-Chavez to make random and arbitrary demands, such as suddenly announcing that students must attend a new course being offered by a visiting colleague.

“He had different requirements for different students, and nobody knew what any of those requirements were,” says Jack Pringle, a second-year graduate student in the program. “As soon as I arrived [in 2018], it was very clear that everyone was terrified of Carlos. But there was also this worship of Carlos. And I think he liked having that power and dependency.”

Banishing imposter syndrome

Castillo-Chavez says such comments, which he says he’s heard for years, mischaracterize his approach to mentoring.

“I think that some students did not understand the rigors of the program, and that its flexibility created challenges that some students were unable to meet,” he says. “I have devoted my life, and sacrificed my family, to this cause. But if you want to stand up and promote diversity and criticize the lack of diversity [at an institution], there will be people who accuse you of wanting to lower academic standards and dilute the quality of the program.” 

One former student, Anarina Murillo, says Castillo-Chavez gave her the courage to pursue a career she thought was beyond her grasp.

“Carlos could tell that I suffered from imposter syndrome,” says Murillo, referring to the feeling of many minority students that they don’t belong and that their shortcomings will be exposed. “So he told me to look in the mirror and say, ‘I’m going to get a math Ph.D.’ He told me to say it over and over until I believed it.” 

Murillo was a junior at ASU when she applied for and was accepted into the MTBI program. “And that’s when I fell in love with mathematical modeling,” she says.

Her academic career took off from there. She earned a BS in 2010 and then ripped through the AMLSS graduate program. After doing a post-doc at the University of Alabama, Birmingham, she accepted Castillo-Chavez’s invitation last year to join him at Brown University. Within months she was hired as an assistant professor in Brown’s School of Public Health, where she applies her math background to the design of clinical pediatric studies.

“I always knew that I wanted to be a professor,” she says. “But I never expected it to happen.”

“Exactly what I needed”

Martinez, the ASU graduate student who later filed a complaint against Castillo-Chavez, hoped he might do the same thing for her academic career, which was in crisis.

In 2011 Martinez was sexually assaulted shortly after starting a PhD program at the University of California, Berkeley. The injuries she suffered eventually required five surgeries and the use of a wheelchair for 18 months. The experience also led her to rethink her decision to pursue a degree in theoretical mathematics. 

Martinez says she was moved by Castillo-Chavez’s passion for helping minority students when she heard him speak at a Latinx in Mathematics conference in Los Angeles in March 2018. And she thought that AMLSS was an ideal setting to pursue her new-found interest in the evolution of societal attitudes toward violence against women.

“This is exactly what I needed,” Martinez remembers saying to herself. “The whole idea behind his program is to apply mathematics to problems in the social sciences. I also needed somebody who would believe in me.”

Martinez says Castillo-Chavez promised her generous funding and “a safe space to learn and work.” But her hopes for a brighter future faded quickly after she arrived in August 2018.

In one case, she says, Castillo-Chavez “became very agitated” and berated her for talking to a reporter who wanted to write about her experience at Berkeley. (No story ever appeared.) He repeatedly cited her ongoing medical issues in conversations about her academic status, she adds. Castillo-Chavez also threatened to renege on a promise to bring her to Brown, she says, “because you’ve been sick so often.”

Martinez says Castillo-Chavez questioned her commitment to the program and her ability to complete her degree. After denying her request to miss 2 days to attend her brother’s graduation from medical school, she says he told her that she was “a bad student… not good at research… undisciplined and a troublemaker.” In her 23 April 2019, complaint to President Crow, Martinez wrote, “Dr. Castillo… created an environment so tense that I am scared to say anything to anyone.” 

Weighty choices

ASU officials declined to discuss any aspect of their investigation. But emails Martinez provided describe how the university’s associate general counsel, Becky Herbst, sought her approval for an “informal resolution” of her complaint in which Castillo-Chavez would agree to step down from his leadership positions and retire. The findings of the investigation were never made public. Martinez accepted that solution, she says, because “he could have hurt so many other people if he remained.”

The fate of the empire that Castillo-Chavez built is “under review,” says Patrick Kenney, dean of the College of Arts and Sciences. “No decisions have been made regarding how will move forward next year and beyond,” Kenney said earlier this month. 

Castillo-Chavez, who has applied for emeritus status, will not have a say in those decisions. But he is worried about what might happen.

“The future of everything I have created is uncertain,” he says. “I’m not sure there’s anybody else who would be willing to do what I’ve had to do to keep it going.” As one faculty member who has clashed with him puts it, “a center without Carlos is not something that anybody has ever considered.” 

The future is equally murky for Martinez, who last year moved back to her hometown of Los Angeles after finding another advisor from a different department. “I’d like to teach math at a community college in the Los Angeles area if campuses reopen in the fall,” she says, noting that she’ll be taking a leave of absence at the end of this semester to weigh her options. It would be a way to give others the chance at a quality education that her parents, who were factory workers, were denied. 

However, she doesn’t know if she’ll ever finish her dissertation and earn a PhD. “I want to be a mathematician,” she says. “But after my experience with Dr. Castillo-Chavez, I don’t know if I can.”

Molly Stellino graduated from ASU this month and served as editor of State Press Magazine. Jeffrey Mervis is a senior correspondent at Science.

Reach the reporters at and or follow @mollystellino and @jeffmervis on Twitter.

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