From the description:
Altruist: First bounty you manually awarded on another person's question
Investor: First bounty you offered on another person's question
What is the difference between these two badges?
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To get the Investor badge you simply need to offer a bounty. You get the badge the day you offer the bounty.
For the Altruist badge, you not only need to offer a bounty, but you need to actually award it before the end of the bounty term.
You can offer a bounty, but not award it. A case in point would be offering a bounty on a question, but still no answers get posted. The bounty would expire without being awarded. You'd get the first badge, but not the second.