Based on a Pentagon report that Secretary of Defense James Mattis summarized in amemo to the president, the new policy overturns the blanket ban on all transgender personnel that President Trump tweeted out last July. But its justification is much the same: allowing transgender people to serve would disrupt unit cohesion, a fundamental building block of combat effectiveness.

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But these statements rely on two myths: that diversity reduces unit cohesion, and that unit cohesion enhances military effectiveness. In fact, there is little evidence for either.

Excluding groups from serving has a long history

Trump’s transgender policy is another installment in a long American tradition of excluding various groups from military service because putting them in uniform supposedly undermined unit cohesion.

Before World War II, the U.S. military maintained racially segregated units and opposed integration of African Americans into white combat outfits. Army Chief of Staff Gen. George Marshall defended this policy in 1940 by claiming that “to intermingle white and colored personnel in the same regimental organization . . . would inevitably have a highly destructive effect on morale — meaning military efficiency.”

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Threats to unit cohesion were also cited as a major reason for maintaining the ban on permitting gays and lesbians to serve openly in the military. And cohesion fears continue to appear in the debate about allowing women to serve in combat units.

Myth No. 1: Diversity harms unit cohesion.

Experience and research debunk the claim that uniformity among members is required for cohesive groups to form. For example, despite widespread hostility to the idea beforehand, a post-World War II Army survey about the experience of units that received black infantry replacements after D-Day found 80 percent of white officers and 96 percent of white NCOs stated that black and white soldiers had gotten along very well or fairly well.

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Other studies have shown that the integration of women, gays and transgender people into the U.S. military — or other militaries around the world — has not adversely affected cohesion, either.

The reason is simple: The heterogeneity of a group’s members is unrelated to its cohesiveness. If nothing else, the dozens of experiments showing that members of groups formed on the flimsiest of pretexts — such as eye color — immediately favor in-group over out-group members should give one pause about the level of uniformity required for group cohesion.

Indeed, much research has shown that diverse individuals can come together and perform effectively as a group if they are committed to achieving a goal that requires cooperation, known as “task cohesion.” In other words, soldiers need not like each other to perform a task well.

Myth No. 2: Cohesion increases military effectiveness.

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The argument that cohesion is crucial to a combat unit’s performance has its roots in an article written shortly after World War II by two University of Chicago sociologists, Edward Shils and Morris Janowitz. Although their argument about primary group cohesion has been tremendously influential in political debates, Shils and Janowitz’s study had numerous problems that undermined its findings.

Shils and Janowitz served in the intelligence section of the psychological warfare division at Supreme Headquarters, Allied Expeditionary Force in Europe. They interviewed German soldiers captured on the Western Front late in the war and asked what motivated them in battle.

Surprisingly few soldiers claimed to be inspired by Nazi ideology. Rather, Shils and Janowitz found that the strong determination with which the Wehrmacht fought was explained by each man’s sense of belonging to his immediate primary group. As Shils and Janowitz wrote of the German soldier, “as long as he felt himself to be a member of his primary group and therefore bound by the expectations and demands of its other members, his soldierly achievement was likely to be good.”

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In short, German soldiers fought for their buddies, not for larger political beliefs such as National Socialism.

But a closer look reveals why this study has fallen out of favor among scholars of military effectiveness. For example, Shils and Janowitz’s interviewees — captives of an enemy that viewed their regime as criminal — had big incentives to play down their Nazi allegiance.

Shils and Janowitz also had a biased sample, consisting of soldiers who surrendered (a rarity among Germans) and fought on the Western Front (80 percent of German casualties occurred in the east while fighting the Soviets).

Furthermore, as the historian Omer Bartov has shown, German casualties were so massive that combat units were quickly annihilated, meaning that primary groups hardly had time to form, much less motivate men to fight.

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Finally, historians, including Bartov and Stephen Fritz, have examined some of the billions of letters written by Wehrmacht soldiers during the war. These letters are filled with references to the negative (fear and disgust of the Judeo-Bolshevist menace) and positive (a magical feeling of national unity) aspects of Nazi ideology, belying Shils and Janowitz’s contention that ideology played little role in motivating German soldiers.

The primary group cohesion thesis also does not have much support in the latest scholarship.

One recent study, for example, argues that soldiers fight with greater determination for authoritarian regimes that, first, inspire them with a compelling national mission (e.g., nationalism, communism, religion) and, second, are able to enforce their unifying ideology with coercion. In this view, regimes like Nazi Germany and the Stalinist Soviet Union produce soldiers who fight the hardest.

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Others contend that democratic soldiers exhibit the most grit in battle because they fight on behalf of governments they view as legitimate.

Still others find that troops perform well in combat when they are so well trained that taking the proper actions under fire becomes second nature. What matters is training and professionalism, not friendship.

None of this scholarship supports the contention that small-unit cohesion improves battlefield performance, and previous integration of excluded groups didn’t hurt either unit cohesion or fighting power. It’s unlikely that the integration of transgender people will, either.