Navigenics Inc., a gene-testing startup, is responding to criticism that the tests could spur bad healthcare choices by teaming up for a broad study of how the results affect behavior. Navigenics charges customers $2,500 to analyze their DNA to assess their risk of developing more than 20 diseases. Several public health officials have said the science on which the tests by Navigenics and other companies are based is too new to be used for making serious medical decisions. Critics fear that some consumers will use positive results to seek treatments they might not need. Negative results, critics say, could inspire others to be less cautious than they should be about lifestyle choices or preventative care. Navigenics is now joining with the Scripps Translational Science Institute in San Diego in hopes of showing those fears are unfounded.