Jewish History: Biblical History: Intermarriage in Egypt:
In Egypt, did the Israelites intermarry with the Egyptians? Is that why their numbers grew so rapidly?
There were certainly instances of Israelite-Egyptian intermarriage (see Lev.24:10), but it doesn't seem to have been common. More likely the Israelites married among themselves or married other Semitic slaves; possibly this was the source of the "mixed multitude" who left Egypt with the Israelites (see Ex. 12:38). "Marrying out" wasn't necessary in order for them to grow in number; all it took was a high birthrate(see Gen.47:27 and Ex.1:7).
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