http://www.fourmilab.ch/etexts/einstein/specrel/www/
ON THE ECTRODYNAMICS OF MOVING BODIES, A. Einstein, 1905: "From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by tv^2/2c^2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B."
Why is the moving clock slow and the stationary one fast? No such asymmetry follows from Einstein's 1905 postulates. What validly follows is that the moving clock is slow as judged from the stationary system, and the stationary clock is slow as judged from the moving system. Einstein's 1905 conclusion (the moving clock "lags behind" the stationary one) is invalid - it does not follow from the postulates.
Einstein knew his 1905 paper was both fraudulent and vulnerable and in 1918 introduced very efficient "general relativity" camouflage:
http://sciliterature.50webs.com/Dialog.htm
Albert Einstein 1918: "A homogenous gravitational field appears, that is directed towards the positive x-axis. Clock U1 is accelerated in the direction of the positive x-axis until it has reached the velocity v, then the gravitational field disappears again. An external force, acting upon U2 in the negative direction of the x-axis prevents U2 from being set in motion by the gravitational field. (...) According to the general theory of relativity, a clock will go faster the higher the gravitational potential of the location where it is located, and during partial process 3 U2 happens to be located at a higher gravitational potential than U1. The calculation shows that this speeding ahead constitutes exactly twice as much as the lagging behind during the partial processes 2 and 4."
http://www.people.fas.harvard.edu/~djmorin/chap11.pdf
Introduction to Classical Mechanics With Problems and Solutions, David Morin, Cambridge University Press, Chapter 11, p. 14: "Twin A stays on the earth, while twin B flies quickly to a distant star and back. (...) For the entire outward and return parts of the trip, B does observe A's clock running slow, but enough strangeness occurs during the turning-around period to make A end up older."
Pentcho Valev