Foreign-born Americans and their descendants have historically been a main driver of population growth, and that role will only increase in the future. The Pew Research Center projects the population will grow to 441 million in 2065, with 88% of the increase is linked to future immigrants and their descendants. This measure also is a reflection of the composition of a community, which can offer insight into its levels of diversity.

In 2017-21, 31% of the population in Brookline was foreign-born, higher than the state rate of 17% and the national rate of 14%. Brookline's percentage increased 4 points from 2000, while the state and nation increased by 5 and 3 points, respectively.

Brookline had the second largest percentage of foreign-born people behind Everett (43%) and was followed by Cambridge (29%), Newton (22%) and Arlington (19%). All localities experienced a growth of 3-5 percentage points since 2000, with the exception of Everett which saw an increase of 21 points. Cambridge had the largest absolute number of foreign-born people, at 34,700, while Brookline, Newton and Everett all had between 19,300 and 20,900 foreign born people.

The multiyear figures are from the Census Bureau's American Community Survey. The bureau combined 5 years of responses to the survey to provide estimates for smaller geographic areas and increase the precision of its estimates. However, because the information came from a survey, the samples responding to the survey were not always large enough to produce reliable results, especially in small geographic areas. CGR has noted on data tables the estimates with relatively large margins of error. Estimates with three asterisks have the largest margins, plus or minus 50% or more of the estimate. Two asterisks mean plus or minus 35%-50%, and one asterisk means plus or minus 20%-35%. For all estimates, the confidence level is 90%, meaning there is 90% probability the true value (if the whole population were surveyed) would be within the margin of error (or confidence interval). The survey provides data on characteristics of the population that used to be collected only during the decennial census. Data for this indicator are released annually in December.