Complex sentence: Lynn argues the surveyed studies have high "reliability" in the sense that different studies give similar results, and high "validity" in the sense that they correlate highly with performance in international studies of achievement in mathematics and science and with national economic development.
Split into simpler sentences: Lynn argues the surveyed studies have high "reliability" in the sense that different studies give similar results. He also argues that they high "validity" in the sense that they correlate highly with performance in international studies of achievement in mathematics and science and with national economic development.