A milestone for research on gender is the use of meta-analysis, which provides a way to assess patterns in a systematic way, which is especially relevant for a topic such as gender, which can be difficult to quantify. Nature and nurture have historically been analyzed when looking at gender in play, as well as reinforcement by peers and parents of typical gender roles and consequently, gender play. Toy companies have often promoted the segregation by gender in toys because it enables them to customize the same toy for each gender, which ultimately doubles their revenue. For example, Legos added more colors to certain sets of toys in the 1990s, including colors commonly attributed to girls such as lavender.
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Puzzles became greatly fashionable as well. In 1893, the English lawyer Angelo John Lewis, writing under the pseudonym of Professor Hoffman, wrote a book called Puzzles Old and New. It contained, amongst other things, more than 40 descriptions of puzzles with secret opening mechanisms. This book grew into a reference work for puzzle games and was very popular at the time. The Tangram puzzle, originally from China, spread to Europe and America in the 19th century.