Research from the University of Texas at El Paso confirms that there is a statistically significant relationship between English and math test scores. Rather than being an artefact of certain individuals testing well generally, genetic and environmental factors influence both sets of skills, causing a correlation.
A correlation means that two things tend to alter in the same direction at the same time, but it does not mean that one thing affects the other. While English and math test scores do show a relationship, this does not necessarily mean that they affect one another directly. There is some research, however, that suggests that strong English teaching also helps pupils with their math scores, while the converse does not seem to hold true.