In the scientific study of human biology, there is no political ideology. Biologists research embryonic development, hormones and sexual maturation to provide scientific evidence that can inform public health.  But the Trump administration’s recent guidance to “restore biological tr uth” on gender determination misses on both “biological” and “truth” and consequently risks excluding tens of thousands of Americans from essential healthcare. Human biology is not black and white. Although newborns are typically categorized as male or female at birth based on visible external genitalia, millions of people are born with biological features reflective of both sexes. These variations arise from the complex ways in which sex develops and takes shape in the human body.  Sex is not fixed at conception; rather, it emerges through a continuous process that begins at fertilization and unfolds throughout gestation. President Trump’s executive order — which is entitled “Defending Women from Gender Ideology Extremism and Restoring Biological Truth to the Federal Government” — ignores this science.  The order recognizes “two sexes, male and female,” defined as those that produce the “small reproductive cell” (sperm) or “large reproductive cell” (egg), respectively. Subsequently, Health Secretary Robert F. Kennedy Jr. announced “guidance” asserting the “immutable and biological nature of sex.”  The scientific inaccuracies within the executive order and Department of Health and Human Services guidance are profound. A quick biology lesson: There are three main sequential steps in making the sexual organs of the human body. First, the chromosomes received at fertilization are analogous to the blueprints for a house, but they are not the house itself. They carry information about whether gonads should develop into ovaries or testes.  Usually, but not always, having two X chromosomes is the blueprint for making ovaries (female), while having an X and a Y chromosome is the blueprint for making testes (male). Second, once testes or ovaries are made in the fetus, their job is to produce hormones that will instruct the body to build sex-specific organs. Hormones travel through the body like construction site supervisors, directing how the “house” should be built. Finally, cells throughout the body function as specialized construction crews, carrying out hormone “construction plans” to build sex-specific cells and organs such as eggs, sperm, uterus, vagina, penis, hair distribution, voice box and musculature. Although these crews typically follow directives without issue, there are instances when they cannot interpret or implement those orders. There are many conditions where the sexual organs of a person do not correspond to the blueprints of the chromosomes. For example, individuals with Androgen Insensitivity Syndrome (AIS) are unable to respond to hormones produced by the testes. As a consequence, they have a female body build, with breasts, female hair distribution and female musculature despite being genetically male. At fertilization, they received a Y-chromosome that directed the development of a pair of testes that make testosterone. However, the body cells — the construction crew — were unable to respond.  So instead of developing into men, they respond to estrogen made elsewhere in the body and develop into women. The testes remain in the abdomen and no sperm are made. This person has a body with both female and male characteristics — outwardly female but born with testes and no uterus.  To say that sex is determined at fertilization is scientifically inaccurate. The making of one's sexual organs begins at fertilization, is executed during gestation, extends through puberty and changes can occur at any stage of construction. The ability to make and respond to hormones differs from person to person, and as a result, humans have a wide variety of sexual body types. Kennedy asserts that the executive order and associated guidance restore “biological truth” to the federal government, yet the non-scientists who crafted these directives ignored the intricate realities of human biology. And now this unscientific justification is being used to eliminate federal funding for medical research.  One letter canceling a National Institutes of Health grant on transgender studies argued that“research programs based on gender identity are often unscientific,” and “many such studies ignore, rather than seriously examine, biological realities.” Given that sex determination is complex and nuanced, health policy must be informed by evidence-based science and grounded in compassion. Researchers and medical professionals work tirelessly to ensure that everyone receives the care they need.  Regrettably, the executive order and Kennedy’s guidance — predicated on scientifically false information — suggest that “gender ideology extremism” exists only within the order itself. Richard Behringer and Carole LaBonne are president and president-elect of the Society for Developmental Biology. ...read more read less