Rationalist origin: I discovered the scientific method in highschool and liked the results of its application to previously awkward social situations, so I extended it to life in general. I came up with most of OB's earlier material by myself under different names, or not quite as well articulated, and this community has helped refine my thoughts and fill in gaps.
Found LW: The FireFox add-on StumbleUpon took me to EY's FAQ about the Meaning of Life on 23 October 2005, along with Max More, Nick Bostrom, Alcor, Sentient Developments, the Transhumanism Wikipedia page, and other resources. From there, to further essays, to the sl4 mailing list, to SIAI, to OB, to LW, where I started interacting with the community in earnest in late January 2010 and achieved 1000 karma in early June 2010. Previous to the StumbleUpon treasure trove, I had been turned off the transhumanist movement by a weird interview of Kurzweil in Wired, but still hopeful due to scifi potentials.
Value and desire to achieve: I'm still working on that. The metaethics sequence was unsatisfactory. In particular, I have problems with our ability to predict the future and what we should value. I'm hoping smarter than human intelligence will have better answers, so I strongly support the Singularity Institute.
If you've recently joined the Less Wrong community, please leave a comment here and introduce yourself. We'd love to know who you are, what you're doing, or how you found us. Tell us how you came to identify as a rationalist, or describe what it is you value and work to achieve.
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You may have noticed that all the posts and all the comments on this site have buttons to vote them up or down, and all the users have "karma" scores which come from the sum of all their comments and posts. Try not to take this too personally. Voting is used mainly to get the most useful comments up to the top of the page where people can see them. It may be difficult to contribute substantially to ongoing conversations when you've just gotten here, and you may even see some of your comments get voted down. Don't be discouraged by this; it happened to many of us. If you've any questions about karma or voting, please feel free to ask here.
If you've come to Less Wrong to teach us about a particular topic, this thread would be a great place to start the conversation, especially until you've worked up enough karma for a top level post. By posting here, and checking the responses, you'll probably get a good read on what, if anything, has already been said here on that topic, what's widely understood and what you might still need to take some time explaining.
A note for theists: you will find LW overtly atheist. We are happy to have you participating but please be aware that other commenters are likely to treat religion as an open-and-shut case. This isn't groupthink; we really, truly have given full consideration to theistic claims and found them to be false. If you'd like to know how we came to this conclusion you may find these related posts a good starting point.
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Welcome to Less Wrong, and we look forward to hearing from you throughout the site.
(Note from MBlume: though my name is at the top of this page, the wording in various parts of the welcome message owes a debt to other LWers who've helped me considerably in working the kinks out)