They need to broaden their understanding of other cultures. 
The broaden-and-build theory of positive emotions suggests that positive emotions (viz. enjoyment/ happiness /joy, and perhaps interest / anticipation) broaden one's awareness and encourage novel, varied, and exploratory thoughts and actions. broaden-extend in scope or range or area; The law was extended to all citizens; widen the range of applications; broaden your horizon; Extend your backyard. extend, widen. increase-make bigger or more; The boss finally increased her salary; The university increased the number of students it admitted. Examples of BROADEN. 1  They need to broaden their understanding of other cultures. 2  The police have broadened the scope of the investigation. 3  Her smile broadened when I told her the good news. 4  The investigation has broadened to include the mayor's staff. 5  His interests broadened to include art and music, not just sports. The broaden-and-build theory is an exploration of the evolved function of positive emotions. It was developed by Barbara Fredrickson starting around 1998 and is commonly associated with positive psychology. The broaden-and-build theory has substantial support. Barbara Fredrickson has conducted randomized controlled lab studies in which participants are randomly assigned to watch films that induce positive emotions such as amusement and contentment, negative emotions such as fear and sadness, or no emotions. 2. broaden-extend in scope or range or area; The law was extended to all citizens; widen the range of applications; broaden your horizon; Extend your backyard. extend, widen. increase-make bigger or more; The boss finally increased her salary; The university increased the number of students it admitted. 1 They need to broaden their understanding of other cultures. 2  The police have broadened the scope of the investigation. 3  Her smile broadened when I told her the good news. 4  The investigation has broadened to include the mayor's staff. 5  His interests broadened to include art and music, not just sports.
https://en.wikipedia.org/wiki/Broaden-and-build http://www.thefreedictionary.com/broaden http://www.merriam-webster.com/dictionary/broaden https://en.wikipedia.org/wiki/Broaden-and-build http://www.thefreedictionary.com/broaden http://www.merriam-webster.com/dictionary/broaden
define broaden