The website M2 asserted that after the first born child the IQ of each subsequently born child drops by 1.5 points. That is, on average the first born is the most intelligent and each successive child born is less intelligent. However, the study has issues that preclude making any such strong claims.
A claim across news sources with varying reputability such as BBC, DailyMail, and Telegraph, were spread quickly through Facebook thanks to over 280,000 shares. The claim originated following a study published by Proceedings of the National Academy of Sciences (PNAS).
While there may be data and research supporting an birth order intelligence correlation, this current article is not strong evidence. The article claims that a 1.5 drop in IQ points is expected per child, except in a two-child family. The idea is that each child "dilutes" the resources of their parents, according to a quote from Dr. Julia Rohrer via the DailyMail. All the articles point out that teaching the younger sibling gives the oldest the upper-hand, but claim that this doesn't necessarily matter in a two-child home.
The original cross-sectional study accounts for children tested between 1969 and 2013. Although having a large sample size is advantageous, the tests used to determine IQ scores varied significantly. For example, in 1969, 11 year-old students were tested individually by teachers with verbal and nonverbal items. This consisted of word association and math problems. In 2013, they used varying ages and presented them with a set of 37 vocabulary words that they needed to define. Scores were then age-adjusted from the number of correct answers. Each age group has a set number of correct answers per "IQ level", each age can be put into another category depending on the number of questions they answer correctly. Not only did the experimenters use varying age ranges, they extrapolated their tests to be equivalent to IQ scores. Most importantly, these intelligence assessments can't be correlated or compared to validated IQ tests. IQ tests ask a variety of questions to fully evaluate an individual's level of intelligence.