Multidisciplinary research in the early twenty-first century suggests that ongoing human evolution could help explain the rise of certain medical conditions such as autism and autoimmune disorders among children. Autism and schizophrenia may be due to genes inherited from the mother and the father that are over-expressed and that fight a tug-of-war in the child's body. Allergies, asthma, and autoimmune disorders appear linked to higher standards of sanitation, which prevent the immune systems of modern humans from being exposed to various parasites and pathogens the way their ancestors' were, making them hypersensitive and more likely to overreact. The human body is not built from a professionally engineered blueprint, but rather is a system shaped over long periods of time by evolution with all kinds of trade-offs and imperfections. Understanding the evolution of the human body can help medical doctors better understand and treat various disorders. Research in evolutionary medicine suggests that diseases are prevalent because natural selection favors reproduction over health and longevity. In addition, biological evolution is slower than cultural evolution and humans evolve more slowly than pathogens. 2015 study found that the frequency of nearsightedness has doubled in the United Kingdom within the last 50 years. Ophthalmologist Steve Schallhorn, chairman of the Optical Express International Medical Advisory Board, noted that research have pointed to a link between the regular use of handheld electronic devices and eyestrain. The American Optometric Association sounded the alarm on a similar vein. According to a spokeswoman, digital eyestrain, or computer vision syndrome, is "rampant, especially as we move toward smaller devices and the prominence of devices increase in our everyday lives." Symptoms include dry and irritated eyes, fatigue, eye strain, blurry vision, difficulty focusing, headaches. However, the syndrome does not cause vision loss or any other permanent damage. In order to alleviate or prevent eyestrain, the Vision Council recommends that people limit screen time, take frequent breaks, adjust screen brightness, change the background from bright colors to gray, increase text sizes, and blinking more often. Parents should not only limit their children's screen time but should also lead by example.